On a sunny afternoon in her Paris apartment, Mireille Bouquet, an assassin-for-hire, receives a peculiar email from Kirika Yuumura, a young student from Japan, inviting her to "make a pilgrimage to the past" together. Initially dismissing the message as nonsense, she changes her mind when it begins playing a song she recognizes from her youth. When they meet in person for the first time, the two are suddenly ambushed by men armed with guns. While Mireille is able to fend off the attack, she discovers that Kirika is quite adept in combat situations after she takes out several gunmen on her own. Despite her proficiency at killing, Kirika laments her inability to feel remorse for her victims. In addition, Kirika apparently has no memory of how she acquired the skills she possesses or even of her real identity. Sensing a shared history between them, Mireille agrees to team up with Kirika, and together, they embark on a journey to uncover the mysteries of their pasts. Operating under the codename "Noir," the duo's fragile alliance is tested as they begin accepting jobs, each one bringing them closer to the truth of who they really are.