On a bright afternoon inside her Paris flat, hired assassin Mireille Bouquet opens an odd email from Kirika Yuumura, a young Japanese student, who invites her to "travel back into the past" together. Mireille first dismisses it as nonsense, but her view shifts when a familiar melody starts playing, one she recalls from her youth.
When they finally meet face to face, gunmen suddenly attack. Mireille quickly defends herself, but she realizes Kirika is surprisingly skilled in combat, eliminating several attackers on her own. Despite her lethal prowess, Kirika confesses she cannot feel remorse for the people she kills. Moreover, she has no recollection of how she gained her abilities or who she truly is.
Recognizing a shared past, Mireille agrees to partner with Kirika. Together they embark on a quest to uncover the secrets of their histories. Operating under the code name 'Noir,' their fragile partnership is tested as they take on assignments, each one drawing them closer to the truth of their identities. As the trail leads them through hidden memories and dangerous encounters, they slowly piece together a past that binds them in ways neither anticipated in depth.