College student Yuuta Hasegawa’s world collapses just days before Christmas when he discovers his girlfriend has been unfaithful. Dejected and wandering through streets filled with happy couples, he literally bumps into Mayu Shinohara, a beautiful underclassman dressed in a Santa costume. Far from offering simple sympathy, the mischievous and bold Mayu becomes fascinated with her gloomy senior and begins inserting herself into his life, finding endless delight in teasing him back to a state of feeling alive. As Christmas approaches, what starts as an annoying nuisance slowly blossoms into a potential new romance, forcing Yuuta to navigate the lingering pain of betrayal while dealing with a girl who is sweet, sassy, and utterly relentless in her pursuit of him. The situation grows more complicated when his ex-girlfriend Reina resurfaces, claiming the cheating incident was a misunderstanding, while his attractive long-time best friend Ayaka also begins to show signs of romantic interest.

A separate narrative follows first-year university student Yuu Isshiki, who is devastated to learn his girlfriend Karen is having an affair with his senior, Kamokura. Instead of succumbing to despair, Yuu approaches the most beautiful woman on campus, Touko Sakurajima, who happens to be Kamokura’s girlfriend. Initially suggesting revenge sex, Yuu instead agrees to a far more calculated plan proposed by Touko. Their scheme involves gathering concrete evidence of the affair and executing a public humiliation on Christmas Eve, where they will simultaneously dump their unfaithful partners and declare they are spending the night together. As they work together, the pair develops a psychological alliance, with Touko coaching Yuu on winning back Karen’s fleeting interest to make the eventual betrayal hurt more, while Yuu helps Touko confront her own insecurities about her attractiveness. The meticulously planned revenge reaches its climax at a circle party where the hotel prize for the best couple becomes the stage for their dramatic confrontation, leading the two betrayed souls to question whether their partnership is simply about vengeance or the beginning of something genuine.