Akuto Sai meets Junko Hattori on the train to the Constant Academy. The two make a powerful pact to make the world a better place, a pact that will mean death if broken. Upon arrival, Akuto is declared to be a future Demon Lord and the entire school begins to fear him. After his introduction in class reaches a zealous climax, a fellow student named Hiroshi Miwa looks up to him, while the rest of the class becomes even more terrified. Eventually, Akuto slips up when he volunteers for "cleaning duty", which is slang in Constant for becoming a "cleaner," an assassin that maintains the school secrets. This infuriates Junko who challenges him to a duel that ends with Akuto accidentally destroying the classroom in a magical explosion. His later attempts to reconcile with Junko at her dorm again launch her into a murderous frenzy, resulting in a chase through the woods. There, Akuto runs into Keena Soga mid-fight, and she resolves to protect him for personal, though unspecified reasons. She is unable to do so, and Akuto again ends the fight with another freak explosion. Immediately after, he is greeted by Korone, an "observer."

Akuto meets with the principal of Constant with Korone in tow. Her testimony absolves him of any evil intent, but the principal has ordered her to stay with Akuto as a bodyguard, and a judge of whether or not he will truly become a Demon Lord. Akuto meets with Fujiko Etou the following day, and she pledges to help him, despite his destiny. She later recommends that they meet in secret in the mountains behind the school, and instructs him on how to disable Korone. En route, Akuto and Korone are attacked by a demon dog, an animal mutated by mana. Akuto is able to absorb the creature's mana and return it to a puppy, a feat that astounds Korone. The puppy picks up the scent of Keena, whose ability to become invisible is revealed. After deactivating Korone, Akuto rendezvous with Fujiko in the mountains, where she asks him to join the school's Disciplinary Committee to help clear his name. As an emergency way to reconcile with Junko, she also gives him a two-shot magical gun that fires medicinal tablets, which she claims will allow the two people shot by it to understand each others' feelings. On the way back, he is tackled by Keena, still fleeing from the puppy, and she is rendered visible (and revealed to be naked) in clear view of the now-reactivated Korone. In exchange for testifying to Korone that Akuto did not try to molest her, Keena requests his friendship, which he agrees to. At the end, Fujiko is shown contacting Junko, falsely claiming that Akuto had threatened her. Fujiko then enters a secret passage in her room, where she converses with her brother's severed head about her plans for Akuto.

Picking up where the previous episode leaves off, we see a touching scene with Fujiko as a child and her brother. The next scene shows Fujiko doubled over her brother coffin as we're told that he died after failing a mission. Because of her brother's apparently shameful death, it is Fujiko's duty to assume the role of head of the Etou family. After the opening credits we're back with Fujiko who's obviously plotting something evil with a classmate. She is also chatting with the reanimated head of her dead brother. He seems to have grave reservations about Fujiko's plans to dominate Sai Akuto. Miwa continues to worship Akuto, this time in the boy's bath house as they enjoy a late night soak. Miwa informs Akuto about the schools underground which, news which seems to interest Akuto. Miwa also mentions that Junko gained her popularity at the school by fighting this underground. Fujiko arranges a meeting between Akuto and Junko the following afternoon as promised in the school underground. Akuto meets with the student council with regard to his application for being the disciplinary comittee representative. Akuto has to deal with Korone in order to meet with Junko, he decides the only way to ensure she's out of the way is by pulling her tail again, which leads to more of the usual ecchi misunderstandings with Keena interrupting to dump her rice cooker on Akuto since she was banned from keeping it in her room, as a gesture and proof of their friendship. The meeting with Junko turns out to be an ambush by members of the underground. Akuto beats his aggressors as soundly as he did Junko but more violently only to reveal her there when the dust settles. This violence of the beatings and Akuto's emotionless attitude are enough to confirm for Junko that Akuto's intentions all along have been evil. She publicly calls Akuto out for punitive justice and together with a large crowd of students chase Akuto around the school. Kei intervenes at the last moment before Junko can complete her punitive justice (probably cutting off Akuto's head as promised on the train). Junko admits that the situation got out of hand and that she was in part responsible for the misunderstanding that prompted the mob to come after Akuto, only to have the mob call her out for turning back to Akuto's side so easily. Korone saves the situation by showering the agree mob of students in medicated rice.

Keena's intervention manages to somehow make Junko and Akuto friends again. But during a class on mana control he knocks her unconscious again. His teacher, Mitsuko, recommends that he go into the Mental Self-Discipline Chamber. Inside the chamber, where people are said to have died, Akuto gets depressed. For some reason Korone and Keena came along, and he's caught between the two of them. However, they find a mysterious map. Keena is intrigued and wonders if it is a treasure map. The next day, Keena copies the map and it ends up all over school. It becomes the talk of the whole academy. But, Fujiko recognizes something on the map.

Akuto, Korone, Hiroshi, and Eiko go to the school's underground labyrinth to find the spot on the map. They encounter a cloaked being who try to stop them with swarms of bats, but is easily defeated. Akuto digs up a plush recording device buried in a tombstone that guides them to a abandoned research facility, being attacked by an armored statue, in which Eiko finds a screen that locates a underground shrine behind a hot spring inside a cave. Meanwhile, Junko, Kena, and Fujiko make their way to the labyrinth. Junko, who fell underground into the hot spring, tells Akuto that Eiko is actually a government spy. A savage wolf comes after them, but they are saved when Ken and Fujiko arrive. Eiko, in tears after seeing him protect Junko, leaves after giving him a key from the shrine. When Fujiko activates the key, it transports them to the headquarters of the student council, who were responsible for the attacks. As it is revealed that the headquarters is really the base of the demon king, a greedy Fujiko uses the plush recording device to awaken the treasured dragon beast Peter Hausen. It is then that the dragon beast displays a record of the memories of Fujiko's brother leading to his death, causing her to collapse thereafter. An enraged Akuto confronts Peter Hausen, soon becoming his new master, despite his wishes.