Haruko Amaya arrives early to accompany childhood friend Takeru Ohyama to his first day at Tenbi Academy. After she excuses herself to prepare for the induction ceremony, Takeru wanders around to take in the view, and witnesses a curious confrontation between two female students before having a run-in with Kodama Himegami. At the ceremony, Kodoma requests a duel with her enemy Takeru, intending to kill him, but Inaho Kushiya intervenes to save her fiancée. All four become roommates living in a triple.

Takeru walks in on Kodoma changing and gets an eyeful. As a result, the four roommates are nearly late for school, earning Huruko a reprimand from student council president Furan Takaki. The day starts with physical exams, so Takeru and Kengo Usui sneak out to peep on the girls. Kodama catches them but does not kill them. When Takeru is examined his element cannot be determined, leaving him without a maken and defenseless in case of a duel, but Inaho vows to protect him. Takeru wants to join the photography club, but Haruko forbids it, telling him instead to consider the kenkeibu (検警部?, investigation/inspection club).

This time Takeru walks in on Haruko. In order to mollify her, he asks for an explanation of the kenkeibu. After learning that its mission is to ensure the fairness of duels and to protect the duelers, Takeru commits to joining it, as do Kogen and Inaho, each for reasons of their own. The new-member welcoming party is held at an onsen, but since it is co-ed everyone wears a swimsuit (this does not stop Takeru and Kogen from ogling their cohorts). The intrusion of a bear provides the chance for everyone to show off their abilities and demonstrate the spirit of maken-ki.

The first years in the Maken Ki are given special lessons, but Takeru gets extremely sore after them. Inaho gives him a massage, and Haruko begins to help too. Himegami tries to decide what the power she felt from Takeru was, and why he shows now power after that. Himegami then has Takeru go on a date with her as her bag carrier, and tries to find out if the mark on his chest was still there. They are attacked by a member attempting to enter the secret organization, Yamigari, but Takeru defeats him after using Blood pointer and kissing Himegami. His friends who were following him

Haruko is annoyed with Takeru: his pratfalls are just too convenient, so he can make his own lunch from now on. Meanwhile, someone is attacking students on the school grounds. Takeru and fellow kenkeibuin Uruchi Minaya are assigned to investigate, but are caught in the spider's web. Haruko comes to their rescue, proving once again that she is Tenbi's strongest.

The return of the rainy season causes Inaho to experience flashbacks to the time when she met Takeru eight years earlier. She wants him to remember it too, but he can't. Despondent, she wanders off in the rain where she discovers a cat that she names Monji (文字?) for the cross on his head. Monji runs off and is saved from rockfall by Takeru, who is searching for Inaho, who saves him in turn. Takeru still can't remember his promise, but he unwittingly repeats it while apologizing, so Inaho is satisfied.

Takeru's long awaited maken is nearly forgotten as Venus arrives and immediately starts to rub fur the wrong way. A grudge match is set: three-on-three volleyball, but things start to get out of hand.

The teen idol has taken a liking to Takeru, and Team Venus is going to support her all the way. That means taking out the competition, namely Haruko, Kodoma and Inaho. In so doing, they let slip their affiliation with Kamigari as well as their unique interpretation of S&M.

The boys are upset that, at the girls' request, swimming classes are to not be co-ed. In keeping with Tenbi tradition, they propose to resolve the issue with a water cavalry battle. The girls agree, even though it means appearing in their swimsuits for the competition itself. No one seems to hold it against him when Takeru cheats, but with the arrival of Minerva Martha, the final member of Venus, he has no chance to enjoy his victory.

While lounging in the shade, Takeru notices an unknown silvery-maned beauty (Minerva). He and Kogen give chase, but stumble instead into a fight. Takeru officiates the duel, but finds his position on the line after clumsily interfering when he fears for the student's safety (a failing grade on his maken practicum doesn't help his case). Minerva, who broke up the fight, proposes an all-night cram session with Takeru and his roommates, which she uses to gather intelligence for Kamigari, revealing for the first time the extent of Kodama's power as well as something of Takeru's.

In order to raise money for summer training camp, the keikenbu members all sign up to work a special event at the maid cafe. The event is very popular and everyone gets a little hyper. An out-of-control prank sends an embarrassed Haruko running out into the street where she is vulnerable to attack, as Kamigari continues to test Takeru.

Haruko is kidnapped, and Takeru goes to save her, but this is a trap. Things are looking grim when Venus arrives.