One night Hana Kayama opens her front door to find Keito Mikita, a young man she has never seen before, standing on her doorstep. Without any preamble, he proposes to her, asking if they can live together as a married couple. Hana is naturally suspicious, but she soon learns that Keito is actually the younger brother of an old friend. Despite her doubts about the proposal, she cannot bring herself to turn away someone who appears to have no place to go, and so they begin cohabitating. This sudden arrangement forces Hana to reconcile with the fact that the boy she once knew has grown into a man carrying a decade of unwavering true love for her. The story traces the development of their unconventional relationship as Hana navigates her own feelings and decides how to respond to a declaration of love that arrived in just zero seconds.