College sophomore Hasegawa Yuuta receives the worst possible holiday surprise just days before Christmas: his girlfriend, Reina Aisaka, has been cheating on him. Dejected and trying to avoid the happy couples that seem to be everywhere, Yuuta stumbles into a chance encounter with Shinohara Mayu, a beautiful and bold first-year student dressed in a Santa costume while handing out flyers. Despite his gloomy mood, Mayu takes an immediate interest in her heartbroken senior, delighting in teasing him and slowly coaxing him out of his shell. She begins appearing everywhere he goes, and what starts as playful mischief quickly develops into something more as the two begin to spend time together.

The story follows Yuuta as he navigates the emotional fallout of betrayal while finding himself unexpectedly drawn to the irrepressible Mayu. His close friend, Ayaka Mino, a fellow second-year student, provides support as he tries to move forward. However, the past proves difficult to leave behind. At a Valentine's party, Yuuta comes face to face with his ex-girlfriend Reina, stirring up old feelings and complications. The situation grows more tangled when, during a hot spring vacation, his longtime friend Ayaka begins to act in ways that suggest her feelings for him might be more than platonic. Set against a backdrop of university life, club activities, and seasonal events, the story explores whether Yuuta can truly trust again and which of the women in his life might hold the key to his future.