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.

Under observation by the government, Akuto is depressed to find out he cannot go to the seaside school on an island. But Korone arranges for him to go by offering to take responsibility for him herself. However, she makes constant attempts to seduce him to let down his guard. Akuto tries to enjoy going to the beach with Junko, but both Kena and Korone interfere when the former two are having fun. Later on, Akuto sees Hiroshi, who tells him about the legend of the demon king on the island, to which Akuto does not believe. The legend says that the revival of the demon king and the dragon will call forth a legendary hero who would defeat them in battle. Meanwhile, Lily Shiraishi, informing Fujiko about the government's plan to use Akuto for its own benefit, says that she will handle this situation on her own.

It is shown that since Korone failed to seduce and manipulate Akuto, she is forced to resign as a surveillance agent. Hiroshi finds out that he is destined to become a future hero. Yukiko Miwa, Hiroshi's younger sister, goes to Akuto to verify if he is truly indeed a demon king. After both Akuto and Hiroshi deny their statuses, Yukiko runs off in tears. Hiroshi later explains that beneath the legendary lake is a shrine inside a cave, where a sword stands upon a rock. A mysterious man, known as Mr. X, awakens a sea cucumber beast emerged from the lake. Yukiko is captured by him, but Hiroshi and Kena try to fight back. However, Hiroshi is pummeled into the lake, while Kena is also captured in the process. Akuto and Junko rescue Yukiko and Kena, but Mr. X sends the beast to destroy the village on the island. While Akuto seems to be overwhelmed by Mr. X's attacks, Korone soon returns to stop Mr. X. Meanwhile, Hiroshi goes to the shrine where the sword lies. He is able to pull the sword out and gain the powers of a hero, deeming himself as Brave, and he uses a plasma-like attack to eradicate the beast. Korone is back to being Akuto's bodyguard, and she admits that she tries to seduce him in order to teach how to develop an intimate relationship with Junko.

Without either of their input, it is decided that an omiai is to be held at Junko's village. Junko, who has feelings for Akuto, is unable to tell him, and so invites him to the village for other reasons. Akuto knows nothing of the marriage interview, but feels responsible for the messes he has caused and decides to go with her. There they meet Yuko Hattori, Junko's younger sister. She is not only revealed as a pop star idol, but she bears a scar on her neck of which was bitten by a demon beast long ago. That night, Junko readies herself and visits his room, where she lets him know why he was invited to the village. This leads to another misunderstanding as several clan members chase Akuto after seeing Junko in tears. As Akuto flees, he is confronted by Eiko, who tells him that the gods have decided Kena as a threat, and orders her clan to eliminate Kena. On school grounds, Kena is saved by Boichiro Yamato, the leader of the Cabinet Intelligence Magic Office, from being assassinated. He only shows interest in making her his own to control the gods, calling her the "principle of identity". Akuto uses his power to transport himself to the school grounds, but he realizes that Yamato surpasses him in strength.

Without either of their input, it's decided that an arranged marriage meeting is to be held between Junko and Akuto at Iga Village. Junko, who has feelings for Akuto, is unable to tell him, and so invites him to the village for other reasons. Akuto knows nothing of the marriage interview, but feels responsible for the messes he's caused and decides to go with her. That night, Junko readies herself and visits his room, but another misunderstanding occurs. As Akuto flees, he is confronted by Teruya Eiko who tells him that the God Suhara has decided Keena is a threat and ordered the Teruya Family to eliminate her. The Demon Lord awakens within an enraged Akuto!

Fujiko appears with a horde of demon beasts, and they all destroy the school grounds. Akuto is unable to heal his wounds from Yamato's attacks. Yamato's subordinate, 2V, goes to Eiko to assemble the Hattori and Teruya clans to join together in an epic battle against the demon king, letting Junko and Yuko to take care of the demon beasts. Fujiko saves Akuto and Kena from Yamato and takes them on a three-headed dog beast towards the student council headquarters. Akuto summons the assistance of Peter Hausen to stop the government's doing, taking flight above the city. While Hiroshi faces Akuto in battle, Junko and Yuko are to seek the device responsible for awakening the dragon beast. An aircraft carrier appears and attacks Akuto. Due to the work of 2V, Akuto is subsequently flattened by it.

Lily stands outside school grounds to repel against attempt of all the clan members to destroy the demon beasts surrounding the area. Yamato arrives in the headquarters and kidnaps Kena away from Fujiko. Eiko uses Junko to protect herself and orders her to take out Lily. Even to when all fails, Lily does her best to take out Eiko. 2V uses his puppets to fight against the clan in order to force Lily on his side. Lily uses her ultimate attack on him, but he manages to escape. When all hope is lost, Akuto is revealed to have survived 2V's attack from the aircraft carrier. Junko draws her family sword, given by her grandmother, in order to side with the demon king in the name of love.

Akuto defeats Hiroshi in the sky, and carries him down to a forest for Yuko to look after him. Meanwhile, Yamato has taken Kena to the shrine and attempts to make the contract with the principle of identity, but Akuto appears and the battle begins in the presence of the gods. Akuto at last defeats Yamato and takes back Kena. He learns that the demon king was created by the gods to make the contract with the principle of identity and destroy humanity. Akuto refuses and declares his intent to kill the gods, causing their defense system to activate. Akuto puts his life on the line to battle the gods, and put an end to the story they have created. Peter Hausen gives Kena a dragon tooth as a memento for his predetermined death. Akuto and Kena reunites with Junko, Korone, Hiroshi, Fujiko, Lily, and Yuko. Everyone starts a new semester at the academy, in which the battle of the demon king altered the memories of those involved.