Unattractive 31-year-old office worker Tooru Nishimori is married to Mika, the most beautiful woman at their workplace. Due to her extreme popularity with men, some would say that she is wasted on a person like Tooru. Nonetheless, Tooru and Mika could not be happier with their marriage. However, there is one thing that deeply troubles Tooru: the fact that Mika dated multiple men before him, in contrast to himself, whose first and only partner has been Mika. Tooru becomes even more bothered when he imagines his wife having sexual intercourse with her previous partners.

After Tooru dies upon being hit by a train, he ends up reincarnated as his former high school self. He wakes up in front of a high school version of Mika, along with the first person she had sex with. Having travelled back in time, Tooru is now on a mission to protect Mika's virginity, all for the sake of being her first partner.