Tsugumomo draws on Japanese folklore, where a tsukumogami is an object that has gained a soul and become alive and self-aware. Within this setting, the story distinguishes between the mature tsugumomo, who have developed through long years of harmony with their owners, and the aberrant amasogi, premature spirits that are born only to grant the destructive wishes of certain people.
Kazuya Kagami has never gone without his mother's obi after her death. At home and at school, he keeps it safe with him at all times, treating it as a treasured keepsake rather than an ordinary accessory. One day, he nearly loses his life when a wig amasogi attacks him. Just when everything seems to be over, the obi he has protected for so long defends him and transforms into a beautiful girl. She introduces herself as Kiriha, a tsugumomo owned by Kazuya's mother.
With Kiriha's arrival, Kazuya is pulled into a hidden reality he has never seen before, a world shared by gods and tsukumogami. As he adapts to this strange existence, the bond between Kazuya, Kiriha, and the relic tied to his mother opens the door to conflicts far beyond his ordinary life. What begins as an encounter with a guardian spirit becomes a larger struggle that forces Kazuya to navigate the rules of the spirit world, the mysteries surrounding his family, and the power hidden within the keepsake he refuses to part with.