Yako's father was murdered in a locked room. She then meets a man named Neuro Nougami who is a demon from Hell who eats "riddles." He forces Yako to act as a detective so he can solve mysteries. Neuro longs to eat the ultimate mystery.
Sai captures Yako as bait, while she is trying to rescue her mother, for Neuro to come to her rescue. He ties her up at an altar which is used to seal up demon powers.
Neuro is stabbed by spears over an altar that drains his power, and Sai is about to cut him open until Yako stops him. Yako reveals she has figured out who Sai was. Sai becomes shocked by the truth. An earthquake shakes the altar and Neuro is reawakened to full power. He reveals the altar is an entrance to hell and is now opening. Sai collapses from exhaustion after losing to Neuro. Everyone leaves the pyramid as the ground has become unstable with the opening of hell. Neuro says goodbye to Yako as he flies towards the portal and uses all his powers to seal the gate. Yako notes that all has returned to normal, though Neuro hasn't come back. The episode's "epilogue" is after a short time skip; Sasazuka and Ishigaki are at a crime scene, and Ishigaki notes that it's much quieter now that the detectives aren't around. Yako is seen at the now-barren office, having finished packing all of the mystery-related items away. Akane is revealed to be "dead" again because of Neuro's lack of demonic influence. Godai enters the office and says a sad but gruff goodbye, now that Neuro isn't there to enslave him anymore. Yako resigns herself to a life without Neuro, when two men enter the office, one of them begging Yako to save him from death before being yanked away. Yako, confused, muses over what happened at the door, while, in the background, colour returns to Akane's white body. A hand reaches out and grabs Yako's head, and Yako, though in pain, delightedly turns around to see Neuro, returned, on the ceiling, the scent of mystery on the tip of his tongue.
Neuro and Yako take on a murder case with zero witnesses, even if the crime scene was in a crowded street crossing, after the third victim was found. With the help of Yako's friend, Kanae, they discover that the victims have one thing in common, Links, a community site. In the scene of the crime, they meet Hime, the one who leads Links. By acquring a security tape through Godai which shows that Hime was present the day of the crime, Neuro makes Yako join the site to use her as bait by writing in her profile that she knows who the culprit is. The next day, Yako shows up in the intersection, walking by herself. A crowded group surrounds her, and right in the middle Hime appears before Yako in order to kill her; but at the last minute Godai shows up disguised, takes the blow, and is wounded. Because they couldn't tape the moment that Hime hurt Godai, they call her to the office and trick her to admit she is the murderer. She confesses, and tries to kill Neuro, but he stops her with his demonic power and eats her mystery. The episode ends with Yako leaving Links for good.
While in the office, Yako finds a body cemented in a wall with a ponytail sticking out. Because of Neuro's demonic influence, the body regains "life" and is able to move only through her hair. Thus, Neuro hires the hair who calls herself as Akane, promising that someday she will eat the mystery of her murder as well. Later on, Yako goes with Kanae to a comedy show that was giving out free bentos so as to attract more people, where people start to die from laughing due to an unknown reason. Only a few manage to survive, including Yako and Kanae. The police, Sasazuka, Ishigaki and Neuro later show up, and with the help of Akane, Neuro discovers that the criminal was the man in charge of the lights, an ex-comedian, killing everyone by freezing poison in the roof and dripping into everybody's bentos for the sake of a "killer joke". He shoots at Neuro with Ishigaki's gun due to his claim that he was no comedian, yet he manages to survive and eats his mystery. Yako watches from far how the culprit is being arrested, and longs for the day Neuro will solve the mystery of her father's murder.
There's a mysterious bomber in the city who leaves their calling card behind. In the scene of the crime, Neuro and Yako meet Usui, Sasazuka's "boss" who is skeptical of Yako's deduction ability. Despite of Usui's threats, Neuro and Yako take matters into their own hands and find out the pattern of the bomber of destroying buildings. As soon as they know the next building to be targeted, they go in and de-activate the bomb that was in the Women's bathroom, so as to lure the bomber. They are successful, and catch the criminal, a woman who reveals herself as being "Histerrier," and that she bombed buildings due to the frustation of not being able to be herself around her family. After de-activating a second bomb with Sasazuka's help, Neuro eats the mystery and "Histerrier" is arrested. However, towards the end of the episode, Yako is told that the way Sasazuka stopped the bomb (shooting it with perfect accuracy) was a method not taught by the police.
Tells of how Neuro and Yako got hold of the office they're using at present and of how they met Godai. Being tired and frustated of having to summon Yako every time there was a mystery, Neuro decides that it would be convenient to have a place where Yako can be all the time, and therefore decides to have an office. As they are walking down the street, Neuro senses a mystery in a building, and so find themselves in the office of a group of Yakuza (from which Godai was a former member) where the mystery was residing. After learning that the previous boss of the office had been killed, Neuro proposes to solve the mystery in exchange for the office. After questioning the current members and investigating the room, he finds out the murderer was the new boss, who killed his superior in order to be "at the center of their nest." However, with the use of Neuro's demonic power he proves that he was no more than a chicken, and the culprit ran away in fear. And so, reluctantly, the Yakuza handed over their office (after being "persuaded" by Neuro).
A murderer has been leaving behind women's heads in the park bathroom with styled hair caked in blood. Because of Yako's concern and Akane's understanding of the slaughtered "hair," Yako asks Neuro to solve this mystery. However, as he does not sense a puzzle in this one, he replies that Yako & Akane should solve the mystery by themselves if they want it so much, remarking that it would be a good opportunity for Yako to "evolve." By fusing Akane with Yako, the latter gains beautiful hair with which she can lure the culprit. After learning from Ishigaki & Sasazuka the identity of the current suspect (the owner of a popular Hair Salon with a criminal record name), she goes to confront him by herself as Neuro is investigating the scene of the crime. As he finds her, the hair stylist is mesmerized by Akane and asks Yako to go to his apartment. As they arrive, he accuses her of knowing his true identity and restrains both her and Akane so as to slice her head off and style her head. Before Yako can be harmed, however, Neuro arrives and rescues her as he eats the culprit's mystery with Akane's help. As he leaves the apartment Neuro points out his disappointment at Yako not being able to evolve.
Two police are on nightly patrol when they come across a red box, unaware that it is the last remaining pieces of a gruesome murder. It appears to be the work of a well known criminal, dubbed 'Phantom Theif Sai'. Yako, Neuro and the police encounter 'Copy Cat'; an obsessive fan of Sai. The police attempt to arrest him, but Neuro knows who the real criminal is. After being stabbed in the chest, the real Sai appears before them for the first time and makes quick work of Copy Cat. Sai then agrees to go with the police, only to kill them moments later. Sai tells Neuro that he has perked his interests, and will certainly be back. The episode ends with Sai disappearing into the rain and Yako pondering his sad existence.
A mystery about a singer, Aya Eishi, whose manager and producer have been murdered long ago. She comes by the office to ask for Yako and Neuro's help to find the killer. She explains the police have written it off as suicides because they were hung at times when nobody could've come to kill them. Aya explains that her songs are aimed at people who are lonely and that they sold so well because of it. At Aya's office she has been sent pictures of herself covered in red paint or blood. Yako thinks the stalker is the one who committed the crimes. The stalker stops Aya while she is underground heading to her car. He is revealed to be a bald man who strangles himself for pleasure with strings pulling the skin on his skull together. He says he will handcuff Aya and take her to a room where he will pleasure her and wets his pants while saying this. Aya is revealed to be Yako in disguise and the Stalker is taken in custody...
Aya thanks the detectives and Yako for solving the case. Neuro says the stalker was not the murderer. During a live concert, Neuro throws Yako onto the stage to point that Aya is the murderer. Aya admits to it and says that she must be lonely to make songs for lonely people. She is taken in without a fight with fans cheering for her. Yako's fame has grown greatly now that she has appeared on television.
There's a jealous actress on the loose, and she's not afraid to kill all the other cast members to achieve her dreams of stardom. Luckily, a certain demon detective shows himself in order to foil her plans.
A family conspiracy surrounding a statue. Sai makes his second appearance and critically injures Neuro.
Neuro and Sai face off. Sai is determined to see the insides of a demon.
Yako receives tickets to an onsen, during the trip a murder occurs.
The first victims infected by the "Electronic Drug" are set free to cause mayhem. Neuro won't let this mystery get away so easily and hunts down the source, HAL.
Neuro loses to HAL's 3 Sphinxes in the Digital World. He figures out the Sphinx are data uploaded from supercomputers located in the City. Neuro finds one at a research facility and destroys it. HAL uses the drug on Yūya Higuchi who helps him create a new Electronic drug that takes control of people faster, but they stay under control shorter if they are unexposed to the drug. The second Sphinx is placed on a truck driven by Yūya Higuchi. Neuro chases him. Yūya Higuchi uses a bridge of people to cross a chasm and Neuro crashes into the gap.
Troy, a murderous desk, has been causing problems for all his owners. Neuro sidetracks from Sai in order to eat this smaller mystery.
Yako is curious about her mother's whereabouts and makes plans to find her. Meanwhile, Neuro's latest enemey is a frog?
Yako hunts down her mother and Neuro is forced to go with her. Some how, the police department all followed...
Neuro, Yako and the detectives from the police are called to a restaurant that is famous for its food bringing glory and success to anyone who eats it. They have been asked to solve a case revolving around the disturbing letters being sent to the Head Chef, Unno Kouji. As soon as they arrive, however, Neuro predicts that the real mystery is about to happen. They are greeted by the owner Shirota Masakage, who after briefly introducing them to the Head Chef, takes them to his office where he shows the detectives the two threatening letters the restaurant received. Mere moments later, Unno is discovered dead in the kitchen. There are no suspects due to the circumstances. However, when Yako tastes one of Shirota's dishes, regarding it as "not food", Neuro becomes interested in him. With the help of the 777 Tools of the Demon World and his reasoning, he discovers that Shirota was the murderer and eats his mystery. He is arrested for murder and for cooking with the use of drugs (the source of glory and success for his food).
The crew visit a lake where the scent of mystery thickens. This may have something to do with a local legend, a corrupt government and Godai's junk.
It turns out before Neuro crashed, he threw Yako safely above the bridge. Neuro is surrounded by electronically-controlled people. He battles them off until Higuchi activities speakers to emit sound to power up the controlled humans. Higuchi explains why he wants the world to be full of criminals. Yako realizes he's not completely controled by the drug and takes the iPod that controls the speakers to disable them. Neuro uses a 7 tool of the demon emperor Evil Aqua to defeat all the humans. Neuro tries to get to HAL this time only to find him protected by a password. Neuro collapses from exhaustion and Yako must find the password. Meanwhile HAL takes over an aircraft carrier and orders supercomputers to be brought on. The final Sphinx has evolved... and Yako has an idea of what the password is.
Neuro, Yako, and Godai arrive to the tanker by a helicopter which is blown up in the process. Neuro enters the digital world of the tanker to pursue HAL. Godai destroys the Sphinx System so Neuro is able to go further in. There HAL is behind his final barrier, a 20 key long password which if typed incorrectly will send nuclear missiles from the tanker and destroy Japan. Yako enters the password and the final barrier is destroyed. Neuro immobilizes HAL and eats up the puzzle. In the real world, the Electronic Drug-controlled Humans break in through the window. Neuro greatly powered by the puzzle commences to defeat (not kill) every single person (20,000 people) on the boat. Meanwhile HAL explains his motive and gives Yako the vaccination for the Electronic Drug. His last request is for him to be deleted before his backup data boots up and make him continue his plan. When HAL reaches 0.00000000001% data remaining, he finds his dead lover in the world between one and zero before disappearing.
Maybe getting plastic surgery isn't such a good thing.