On the last day of summer vacation, high school student Riya spends time with her six childhood friends: Yosuke, Nao, Yuina, Karen, Suguru, and Kazuma. As they part ways, a suspicious app called Happiness installs itself on all of their phones without permission. The app claims to grant any wish, but in exchange, it consumes points that are shared among the group. The group starts with 300 points, with each wish costing a certain number of points depending on its difficulty. If the points reach zero, the app automatically charges another 300 points, but the cost is five years of life taken equally from all seven members.

Riya, the narrator and viewpoint character, harbors secret feelings for Yosuke, the group's easygoing leader. Nao is quiet and mysterious, the only member who initially refuses to make a wish and instead gives his points to the others. Yuina is a hardworking, gyaru-type girl who works multiple jobs and spends money on game microtransactions and cosmetics. Karen is a wealthy crybaby who immediately uses the app for cosmetic surgery to change her appearance. Suguru is an aspiring streamer with an energetic, deferential personality. Kazuma is the stoic athlete of the group, on a sugar-free diet to maintain his physique, who tests the app with a small wish for a hamburger set.

The narrative tests whether the app becomes a weapon that destroys their friendship or a magic that brings them happiness. As wishes are made and secrets revealed, the characters confront whether they can remain friends after seeing each others hidden desires and true faces. The central tension revolves around choosing between oneself and ones precious friends, and whether friendship can survive when everyone has something to hide.