Mirei Tsubaki and Kyohei live together as a single-mother family. Kyohei is the son of Mirei and a man with whom she once had an affair, but his father never acknowledged him, leaving no familial bond between them. Mirei loves Kyohei as her own child, and Kyohei deeply cherishes his mother in return. One day, Kyohei's biological father suddenly appears and declares his intention to take Kyohei away as his legitimate son. Mirei naturally refuses, but the man persists, sparking a fierce argument between them. Overcome by intense emotion, Mirei ends up stabbing the man. Now burdened with guilt over her crime, she must face the uncertain future she and Kyohei will share.