From MangaHelpers:
When the two were still young, Saito and Kawana Subaru were close friends. But with Subaru coming from a wealthy family, she was sent to the top school while Saito had to settle for a regular education.

But after painfully studying for years, he was finally accepted as a high school freshman through a scholarship program that only a handful of students are awarded. But what Saito thought was going to be a heartwarming reunion turned out horrible. Subaru had grown up beautiful and was the most popular person in school as she was kind to everyone, all except towards Saito who she seemed to despise.

Almost regretting his decision to come to the school, Saito wishes that somehow their dorm rooms would connect to each other. Magically, because it turns out that Subaru wished the same thing, their wish comes true but, in return, Subaru loses her self-control for one hour per day. During that time, her erotic desire for Saito, that she can normally hide, comes pouring out and her true intentions are revealed.

He wants to take advantage of this once in a lifetime chance but doing so might jeopardize their relationship during the other 23 hours.