Momoko, a strange girl skilled in the martial arts, locates Kōshi Inuzuka and wants to marry him in order to produce a strong child.
Momoko joins Kōshi's class, much to his surprise. The gym leader, jealous of Kōshi's popularity with a lot of the female students, challenges Momoko to athletic contests with Kōshi's credit in the class on the line.
Kōshi and Momoko go to the zoo together, though before long, animals escape their cages and a massive polar bear attacks them. All of this is a plot by Iroha to kill Kōshi.
Iroha tries repeatedly to assassinate Kōshi, but upon developing a crush on him, she is unable to go through with it.
Iroha tries once again to kill Kōshi but gets caught up in the moment when Kōshi and Momoko are showing her around the park. Eventually, Hanzō has to step in to try to complete the mission.
Tenka Koganei of the Tiger family comes to assassinate Kōshi, but ends up in a fight to the death for Momoko's affection.
After learning about the meaning between their marriage from his father, Kōshi starts to act nicer towards Momoko though soon worries about being manipulated and distances himself from her. Eventually, Kōshi realizes how much he has hurt her and tries to rectify for his actions.
Iroha and Hanzō join Kōshi's school and instantly Iroha notices how friendly he and Sanae are to each other. Iroha proceeds to help Momoko convey her feelings more strongly to Kōshi.
Sanae, as a member of the horse family of the Zodiac Masters, is bound to protect Kōshi from danger without revealing herself to anyone. She is constantly battling the feelings she has for him.
Sanae's fiancé tries to prove to her through various strength tests to show her that he is worthy of becoming her husband. Meanwhile, Sanae is having enough trouble just keeping her martial arts background a secret from Kōshi.
When Hikaru, Sanae's supposed fiancé, comes looking for a duel with Kōshi at school, Sanae must endure intense embarrassment in order to save him.
Both Kōshi and Hanzō come down with a cold, though Hanzō is almost completely ignored by Iroha in favor of Kōshi. Hanzō gets very jealous of Kōshi and starts tormenting him by impersonating him and others in order to harm Kōshi.
A new female teacher has been hired at Kōshi's school who is very well-endowed and knowledgeable about the law. Before long, Sanae begins to suspect her of being an assassin and attempts to confirm her suspicions.
A girl, thought to be dead due to an accident at the school's cultural festival decades previous, has resurfaced. It is rumored that she died before she got a chance to confess her love to Kōshi's father. Kōshi hence agrees to date the girl so that she can peacefully rest in heaven.
Saigō meets the girl of his dreams after he thought he protected her from thugs. The girl is actually a martial artist coming from a tribe known for training assassins. The girl is given a choice: Kill Kōshi, or Saigō will be killed.
Momoko gets some advice from a television program about how to make Kōshi notice her more, but her initial attempts fail. Iroha and Hanzō try to help out as well, but with no much success. At the end of the episode, Momoko and Kōshi find a disheveled Tenka.
Tenka is taken back to Kōshi's house where he is given nourishment. He informs Kōshi and Momoko that his siblings are coming to kill Kōshi. Tenka's younger brother arrives to try to assassinate Kōshi, and Tenka attempts to stop him.
Kōshi's house has burned down and he must now move into the same apartment building as Iroha and Hanzō; Tenka lives there too. As a welcoming party, the landlady hosts a special welcoming party involving nabemono.
A woman claiming to have been fired from being employed as a maid arrives at the apartment building. where Kōshi is currently staying. She initially comes off as a kind and helpful person but soon reveals to Momoko that she is an assassin who has come to kill Kōshi. Momoko tries to plead her case to the others, but no one will believe her.
Kōshi was made to swallow a ball of poison from the Tenga clan and was told that in a week he will die unless he is given an antidote. Momoko willingly swallows the same poison so that if Kōshi dies, she does as well. Momoko and the other martial artists on their side train for a week, and Kōshi tries to learn a technique of his own, but fails. Kōshi and the others then go to the Tenga clan's main house and must fight several battles to gain the antidote.
With the first battle won, Iroha and Hanzō face up against Tenten in the second match. After a little while, Tenten starts to tease Iroha by revealing to Kōshi and Momoko that Iroha is in love with Kōshi. This in effect makes her enraged to the point where she cannot battle properly and is defeated. Hanzō tries to turn things around which gives Iroha the last bit of strength she needed to give one last powerful attack, though Tenten easily countered it leaving Momoko to intercept Tenten's last attack before it hit Iroha. It turns out Tenten is in love with Hanzō, though does not reveal this. Instead, she gives up her portion of the antidote and walks off. This leaves Kōshi and Momoko to go on ahead to the final battle.
With Kōshi and Momoko poisoned, they have to still battle Tenga Koganei in order to get the antidote, but the sun is close to setting, which means death for Kōshi and Momoko. Upon entering battle, Momoko finds that Tenga is much more powerful than she expected and before long the battle is all but over. When Momoko cannot fight any longer, Tenga gives the antidote to Kōshi, though there is only enough for one person. Right before the sun sets, Kōshi gives Momoko the antidote, and after she awakens to find Kōshi dead, unleashes her ultimate fury on Tenga, and wins the battle. Afterwards, it is found that Kōshi was miraculously healed due to a martial arts technique his mother had performed on him when he was young. Everyone goes back to living their "normal" lives.