The following is a summary of the plot of the manga series. For the anime version refer to Plot of the series (anime).
Prologue[]
Meeting Neuro and the first case[]
Chapter 1
We are first introduced to the title character of the series, Neuro Nōgami, a demon who consumes a type of energy called mysteries. Having exhausted the Demon World of its supply Neuro decides to go to the Human World to search for the finest and tastiest mystery since the ones he had in the Demon World were bland anyway.
The scene shifts over to the Human World, where we are introduced to Yako Katsuragi, the second main character of the series. She is seen at the funeral of her recently murdered father, Seiichi Katsuragi where she is comforted by her mother (Haruka Katsuragi), cleaning lady (Sonobe Miwako) and the detectives in charge of the investigations (Keitarō Takeda and Eishi Sasazuka). Yako later sits alone in the room with the family altar. She begins weeping over her father's loss until Neuro appears in front of her for the first time and introduces himself as a demon. He decides to get Yako to be a detective so he can solve cases without attracting attention and easily scares her into following him so he could demonstrate what he meant.
They both go to a nearby cafe where a murder occurs. The same detectives from earlier arrive at the scene and Neuro introduces himself to them and informs them of Yako's detective skills. Neuro and the detectives begin their seperate investigations after this, while Yako was told by Neuro to just stand and watch. Neuro activates his first Demon World tool seen in the series; Evil Friday, to assist him in the investigations. Takeda later comes to the conclusion that the criminal was the junior employee of the victim who had been sitting with him and the murder method was by poisoning the victim's sandwich. Neuro rejects this however after doing his investigations,claiming Yako had found the real criminal.
Neuro makes Yako point her finger at the true culprit, the woman sitting next to the washrooms. He explains she had put the poison on the doorknob of the washroom so that the victim would be poisoned after touching the doorknob and the sandwich after that. She gets angry of being accused however, but Neuro counters her by asking her to explain why had she been loitering around the area for 30 minutes before sitting at her seat, which was a smoking area and why didn't she smoke since she sat there. Neuro consumes her mystery after she was unable to respond, making her mystery his first in the Human World. She breaks down after this, explaining the she had a relationship with the victim and her life began to crumble when she found out he had a mistress. Neuro is uninterested however and forces Yako to bring him to her house, where he would solve the next mystery.
Solving Katsuragi Seiichi's murder[]
Chapters 2 to 3
Neuro and Yako later reach Yako's home, where Yako's mother, Haruka greets them. Neuro tells her Yako had been quite popular lately in the crimes and investigations industry, and he had become her assistant. Immediately after this, Detective Takeda and Sasazuka arrive to thank Yako and Neuro for helping to solve the case earlier. Before they leave, Neuro asks them to stay a bit longer as they could solve the murder of Seiichi Katsuragi should they be given thirty minutes. Sasazuka tries to object but Takeda agrees, feeling that this may cheer Yako up by seeing her father's case positively.
Neuro and Yako go to the crime scene, Seiichi's room. Yako asks Neuro why is he so interested in mysteries and Neuro explains they are his source of food and energy, that the mysteries are a form of negative energy formed by emotions such as anger and hate. Neuro begins his investigations after this, using Evil Script to physically enter the computer and steal the information of the crime from the police network. He finds out Seiichi had been in the room by himself during the time of the murder, and he was killed by multiple knife stabs in the locked room. Realising Neuro's power and abilities, Yako pleads to Neuro to solve her father's case. Neuro complies, telling her he would solve the case even if she tried to intervene.
Neuro calls in the others after this, stating the criminal has already been identified and gets Yako to point at the culprit - detective Takeda Keitaro. Neuro explains Takeda had entered the room through the window and placed a modified ruler so that if the window were to be closed the ruler would slide down and create a makeshift lock the prevents the window from being opened. While stabbing Seiichi, some blood landed in his left eye and the contact lens fell off. Unable to find it in the dark, he escaped and locked the window from the outside. Takeda later used his position to get himself assigned as the main detective for investigations of the case. Having the authority to be the first to enter the crime scene, Takeda broke the locked window and hid the ruler before anyone else could notice. He could not find the contact lens however, and hoped someone else who finds it could give it to him and he could pretend he had lost it during investigations. The lens proved to be his undoing however, as Neuro found it covered with dry blood, proving Takeda had been in the room during the crime.
Takeda is unable to explain himself and is angered he was careless enough to allow Yako to solve the case when he could have gotten away with it. Neuro consumes his mystery, and Yako begins berating Takeda and asks him what was the reason he killed Seiichi. Takeda shows his true nature and explains he became an officer because he enjoyed seeing faces of victims or their close ones in agony, and after seeing Yako's joyful expression when she was in a restaurant with her family he became determined to see her negative expressions. Sasazuka calls Takeda mad and tries to arrest him, but Takeda tells Sasazuka to stay back as he wanted to see Yako's agonized face. Neuro intervenes and puts Takeda in a trance however, causing him to see an illusion of a monster behind Yako. He becomes mad and is taken away by the police.
After the arrest, Neuro leaves Yako. Yako and her mother spend a few days together after this grieving and mourning Seiichi's death. Yako finally gets over it and is able to sleep peacefully again after this, jokingly admitting she should have taken the detective thing more seriously that day. Neuro suddenly pops in her room at this exact moment and tells her she would continue her role as the fake detective with him as the assistant, and they begin their alliance that would last for the rest of the series.
The Supreme S. case[]
Chapters 4 to 6
A few days after the incident, Yako is seen taking a stroll around town with her best friend, Kanae Kagohara. They are interrupted when Neuro sends a message to Yako's handphone, demanding her to meet him to solve a mystery. When she arrives, she finds Neuro just done taking a car apart, wanting to eat the mystery of the car's structure though it did not satisfy him. Neuro brings Yako to the potential crime scene, where they encounter detective Sasazuka from the previous cases with his new partner, detective Jun Ishigaki. Ishigaki finds Yako and Neuro's behaviour suspicious but Sasazuka brushes it off and lets them join him.
They arrive at the Supreme S., a famous restaurant known for its specialty, the victory dish. Upon their entrance, Neuro correctly predicts a murder happening in the restaurant though the killing takes place offscreen and the main characters weren't aware it had taken place. They are greeted by the head chef, Shirota Masakage. He explains the restaurant had recently been receiving letters threatening them to end their business. Their discussion is interrupted however when one of the chefs discovers the dead body of the assistant head chef, Unno Kouji in the kitchen.
After Unno's body is removed from the crime scene, a devastated Shirota and the others begin discussing over his death. The cause of death was identified as a single blunt attack to the back of the head delivered with extraodinary strength. According to the witnesses, noone had entered his kitchen through the front so they assumed the murderer came and left through the back door. Shirota asks them to find the culprit and the detectives and Neuro begin snooping for clues. Yako finds one of the restaurants' specialty victory dish and Shirota, noticing her eagerness and excitedness gives her permission to have it. After having a bite however Yako notes a certain disatisfaction she felt from it. Shirota is angered that she dared to insult his dish and returns to his kitchen to get back to work and Jun forces Yako and Neuro out for disrupting investigations. Outside, Neuro uses one of his demon tools, Evil Javelin, to discover the contents of the victory dish Yako had just eaten. It turns out the victory dish was laced with energy enhacing drugs such as cocaine, and Neuro, having collected all the required evidence decides to reveal the scoundrel behind the case to consume his mystery.
After gathering everyone, Neuro gets Yako to reveal the criminal as none other than Shirota Masakage himself. Neuro explains that Shirota had murdered Unno before they arrived and he basically kept the body standing upright by supporting it with two ropes connected with a frozen tomato. The steam from the boiling pot below the body eventually thawed the tomato and the body would topple over after the ropes break through the softened tomato. Neuro then explains the first threat letter had been written by Unno, who wanted the restaurant to cease its business due to its true nature. Shirota realized how much suspicion the letter may cause and prompty wrote a second one in a similar style to make it look like the original writer simple had a grudge against the restaurant. After this, Neuro proceeds to reveal the murder weapon, which was a bloodied rolling pin Shirota had hid in a rush earlier.
Realising he was already cornered, Shirota explains the reason he killed Unno was because Unno planned to report the truth of the victory dishes just when he was getting close to making his ultimate dish. After admitting to his crime, Shirota reveals his ultimate dish and injects his arm with the soup, transforming him into a musclebound monstrosity. Neuro begins provoking Shirota and is attacked, though Neuro immediately suppresses him and consumes his mystery, draining him of his energy in the process. Shirota is arrested later, and the restaurant's secret is revealed through the media.
Obtaining an office[]
Chapters 7 to 10
After the Supreme S. case, Yako is having a meal with Kanae when Neuro interrupts her again to find some mysteries. Yako expresses her annoyance at Neuro constantly getting in the way of her daily life and Neuro agrees it was getting somewhat troublesome since he had to look for her all the time. He decides they would get an office as a hub of sorts so that they could have a set place to meet up and the customers with mysteries would come to them once they begin their detective business. They begin looking aroujnd town for a suitable place when Neuro senses a mystery in a building nearby. Neuro drags Yako in to investigate when they come across a shady finance company which was emitting the mystery.
Upon their entrance, they are roughly greeted by the company's acting vice-president, Godai Shinobu, who tells them to go away. Before Yako could scamper away however, Neuro states she would like to solve the incident that had taken place in the office, surprising Godai and the rest of his group as they had not planned to let any outsiders know. The company's acting president, Washio Masakatsu decides not to makes things worse and tells them their boss, Kuniharu Saotome, had been murdered 3 days ago. Neuro states Yako plans to solve the murder for them and find the culprit. Godai immediately objects and points a knife at Yako, only to be knocked around the room by Neuro (who made it look like Yako did the deed). Washio notices this and relunctantly allows them to do their investigations. After this,Neuro and Godai's gang strike a deal where if Neuro solves the case, the office would be put under his and Yako's ownership whereas if he fails to solve the case they could have Yako do anything for them (much to Yako's chagrin). Yako becomes nervous and asks Neuro how could he be so confident in solving the case and Neuro tells her not to underestimate him.
Washio goes into detail on how the whole incident started after this. Saotome had been working late on the inght of the murder since Godai had messed up some paperwork. Meanwhile, Godai, Washio and the other three decided to go to the bar and wait for Saotome to join them. Saotome did not show up and he did not answer his cellphone however, so Washio decided to go check on him. The others joined him later when they saw Washio knocking on the office door, saying it was locked and there was no response from the inside. Godai kicked the door open and they discovered Saotome's head severed off the body with a clean cut, assumingly from a sword or blade.
After this, Washio and the other 3 employees of the office began lamenting their boss death, saying he had done so much for them and since they worked in the underground business the murderer could've bee.n anyone who kept a grudge against him. Godai gets up and starts insulting the late boss however, saying he always gave a low salary and how much better it was without him, causing the group to begin quarreling amongst themselves. Neuro decides to begin investigation and uses his Demon World tool, Evil Blind, to conceal himself and Yako. After the tool's effects wear off, Neuro tells Godai's gang Yako had solved the case already and he would be leaving for a while to get some supplies to demonstrate the murder method, leaving Yako alone in the office with the group.
The tension grows high in the office for Yako after this as she sits with Godai while the other four employees began planning what to do with her. Meanwhile, Neuro encounters detective Sasazuka and Ishigaki outside after getting the supplies. Sasazuka asks Neuro if anything is going on, but Neuro decides not to tell and walks off. Back to Yako's situation, Yako and Godai were having a little chat on how bad things were for life in the underground business and how worthless they were ot the higher-ups. After reminiscing a bit on his time with Saotome in the past, Yako comes to the conclusion that Godai could not have been the culprit despite all the things he said earlier. Neuro suddenly returns and interrupts them to declare he was ready to demonstrate how the murder was done.
First, Neuro shows the presence of hooks on each corner of the ceiling. He ties a thin wire to each of the hooks and asks Yako to sit at the president's table while holding up the watermelon he bought earlier. After this, he pulls the rope's end at the window and the wire immediately yanked off the hooks, shooting itself towards the window and slicing the watermelon in the process. Neuro states Saotome was beheaded the same way the watermelon was sliced : by having the wire catapult itself to his neck. To support his theory, Neuro shows the bloodstains left outside the window as the wire would be yanked out the window after slicing off the head.
Before he can go on however, Neuro is suddenly shot in the eye by Washio, who says if they allow Neuro to continue he may decide to report the incident to the authorities. Neuro stands up unharmed though, and reveals Washio as Saotome's murderer. Washio, feeling cornered after Neuro proves he (Washio) was the only one who could have done the deed, explains he had always had a sort of inferiority complex where he could not accept anyone else being higher than him and admits he had killed Saotome for this reason alone. Washio begins thanking Godai for inadvertably keeping Saotome working late that night for his plan to work, much to Godai's disgust. Neuro suddenly intervenes and punishes Washio by terrifying and driving him out of the office.
With the case now solved, Neuro reminds Godai and the three employees of their deal where they were to hand over the office. The men become furious and attempt to chase Neuro and Yako out, but Neuro makes short work out of them and scare them out, though he leaves one of his Demon Tools, Evil Stalker to keep an eye on the men in case any may come in handy later. With the whole thing now wrapped up, Neuro proudly establishes Katsuragi Yako's Demon World Detective Office.
The Aya Asia Mystery[]
The Initial Suspect[]
Chapters 11 to 13
Shortly after getting the office, Yako and Neuro begin settling in. Yako finds some rather disturbing and questionable things such as packs of starch lying around and a bodyprint of what seems to be a body with its braided hair sticking out. Meanwhile, Neuro is busy collecting information and earth knowledge and readying the business by making a website for it. Yako notices a concert playing on the television and recognizes the singer as Aya Asia. Neuro is intrigued when he finds out Aya has a hypnotic effect in her singing which cause her audience to faint in excitement.
Their first customer enters during the middle of the conversation and while Yako doesn't immediately notice she is revealed to be Aya Asia. Aya requests them to expose the killer of her close friends who were also her producer and manager. Yako attempts to get Aya's case to a professional like Sasazuka but Neuro intervenes and happily accepts the case, confident it would boost the business' reputation. He asks Aya to go into detail on the incidents.
Three years ago, on the last day of a concert tour Aya's music producer, Daijima Takurou was found hanged in the dressing room after Aya's concert. While there were no such motive as to why Daijima would want to suddenly kill himself, there were no evidence or suspects and the police simply dropped the case and ruled it out as suicide. One year after Daijima's death, Aya's friend and manager was found dead the same way Daijima was. The police came up with no evidence again and ruled the case as suicide as well.
After giving the briefing the the incidents, Aya receives a call to return to her office. Neuro, unwillingly to let this opportunity go to waste decides to tag along. Upon their arrival Miki Hideyoshi, one of the studio employees informs Aya some police officers have showed up. The officers are none other than Sasazuka and Ishigaki, who had come to inquire on the stalker who was supposedly stalking Aya recently. Miki shows the visitors and Aya a box containing a dead mutilated cat with a bloodied picture of Aya on it. He explains the stalker had been lurking at the back of the office and ocassionally broke the windows there.
After the explanation, Neuro, who was still more interested in the prior suicide incidents asks Sasazuka for permission to cross examine the cases. Sasazuka agrees and shows Neuro the documents, which contained the exact time the bodies were found and stated that until the bodies were found noone had been seen entering the victims' rooms. Meanwhile in the next room, Aya and Yako are having small talk. Aya asks Yako how it feels to be a detective at such a young age, though Yako begins feeling a bit down since she was pressured into doing all this. Changing the subject, Yako asks Aya how did she achieve the hypnotic power in her songs and how it all worked on the human mind. Aya bluntly replies it was all gained by loneliness. Before they can continue, Neuro interrupts as usual and drags Yako away, determined to have a good idea on what had happened.
Later in the evening, Aya is seen leaving her car after parking in a parking lot. A hooded man suddenly appears and tells Aya it was useless to depend on the police. Removing his hood, he is revealed to be Aya's stalker, sporting a crazed look and a large collection of handcuffs. He attempts to kidnap Aya. Before he can touch her however he is grabbed by Ishigaki and "Aya" reveals herself as Yako in a wig. Neuro, Sasazuka and the real Aya appear from the corner and Neuro explains he had analyzed the stalker's movements and predicted his actions. Feeling cornered, the man attempts to escape, calling out to Aya to join him as his "greatest love". Sasazuka delivers devastating kick to the man and he goes down for the count. The suffering isn't over for the stalker however as Neuro punishes him by applying one of his Demon Tools, Evil Stringer onto the man for interfering in his investigations. Seeing all this, Aya turns away and coldly remarks to the man he could never be her greatest lover as even with him around she would still be able to sing. Yako is confused by Aya's words, and after the stalker's arrest Neuro grabs Yako and states all interferences have been removed and the real case is ready to be solved.
The True Killer[]
Chapters 14 to 15
At the police station the crazed fan is seen being interrogated by Ishigaki and Sasazuka. The man couldn't give any useful information however and Sasazuka concluded there was no way he could've planned out the murders of Daijima and Hibari. Back at the office Yako is busy looking through the case documents. Neuro notices this and asks Yako to give her opinion on the matter, to which Yako replies that she feels Aya is the true culprit. Neuro congratulates Yako on coming to the same conclusion as he had and drags her away to see Aya. When they arrive however Miki informs them Aya is busy in the middle of a performance at the moment. Neuro decides to wait by watching and brings Yako in with him to join the audience. Aya gets on stage and begins singing, causing the members of the audience to faint and Yako to tear up. After Aya ends her song however Neuro tosses Yako onto the stage and gets her to reveal Aya as the criminal. The staff attempt to catch Yako but Aya halts them and asks Yako to explain why did she suspect her as the criminal. Neuro makes up the usual alibi that Yako had stage fright and was unable to talk so he would do it in her place.
He first explains that the crimes had some similarities, among them the murder method, the things present in the crime scene and most importantly both victims were found by Aya. He explains Aya had simply knocked out the victims and while they were unconcsious she tied their necks with a rope which was connected to a polyethelyne container. She would hang the ropes on the ceiling pipes to create a pulley system and left the water hose flowing into the containers. When the containers were full they would hoist the bodies in the air, effectively killing the victims without the need of lifting them up.
Aya is unable to counter and asks to hear the deductions from Yako herself. Yako explains she had been to several fan sites which stated three years ago Aya's world tour concert garnered lukewarm reception until the last day where the entire audience were awed, and that a similar incident occured two years after. Yako then states incidentally the murders occured the same time as those two periods and she came to the conclusion that Aya killed her colleagues as she craved loneliness and that her singing abilities were held back by their friendship. Aya begins lamenting on her time with Daijima and Hibari, up to the day she was forced to kill them.
After this, Aya admits her guilt and states she had no regrets as the death of her close ones had also awakened her true ablilities. Yako isn't fooled by Aya's cold words however and states that if Aya truly had no guilt and she would not have asked for help to solve the case. Aya smiles and states she was glad it was Yako of all people who solved her case.
After Aya's arrest Neuro and Yako are seen going home. Neuro disappointly states Aya's mystery was rather plain as she had surrendered willingly but praises Yako for her abilities and insight which helped solve the case, marking this the first ocassion Neuro is shown to have respect for Yako. He leaves, telling Yako he looks forward to seeing her in the office the next day (otherwise he would tear her limbs off). Back home, Haruka and Miwako are seen reading the newspaper in awe. When Yako asks what is going on she is surprised to see her face on the front page of the news on Aya's arrest.