Souta, a university student, meets a girl named Yuki during a nighttime encounter. Yuki asserts that Souta previously saved her life, though no details of this event emerge. She displays complete unfamiliarity with ordinary outside-world practices, including shopping, monetary transactions, and social norms. The conditions of their first meeting and the exact nature of Souta's past involvement with Yuki remain undefined, resting solely on her claim of his rescue.