Nora Seed's life unravels: her cat dies, she is fired, her relationships are severed. She catalogs her regrets and decides to end her life, taking an overdose of antidepressants.

Nora wakes between life and death in an infinite library. Mrs. Elm, her childhood librarian, explains the rules: each book is a life she could have lived. The Book of Regrets catalogs every choice she wishes she had made differently.

Nora enters the life where she married her ex-fiance Dan and discovers domestic misery. She then tries the life where she went to Australia with Izzy, finding that friendship and sunshine do not solve interior pain.

Nora lives the life where she fulfilled her father's dream and became an Olympic gold medalist, only to find isolation and emptiness. She then enters the life where she stayed in her band The Labyrinths as they became famous, discovering that fame brings addiction and misery.

Nora explores increasingly exotic lives: Arctic researcher, Cambridge philosophy professor, vineyard owner in France. Each offers genuine rewards and genuine suffering. The pattern of entry and return accelerates.

Nora enters a life where she has a daughter named Molly. For the first time, she wants to stay — not for herself but for someone else. But she is an imposter in her own family, and the life dissolves when regret finds her even here.

The Midnight Library begins to destabilize. Books fall from shelves, the lights flicker, the structure groans. Nora realizes the library exists only as long as she remains between life and death — and that she must choose.

Nora chooses her original, imperfect life. The library dissolves. She wakes in her apartment, having survived the overdose, and begins to live with intention for the first time.