One quiet afternoon at her Paris apartment, professional assassin Mireille Bouquet gets a strange email from Kirika Yuumura, a Japanese schoolgirl, who proposes they "make a pilgrimage to the past" together. At first, Mireille dismisses the message as nonsense, but she changes her mind when it starts playing a song she remembers from her childhood.
When the two meet in person for the first time, they are suddenly attacked by armed men. Mireille handles the assault skillfully, but she is surprised to find Kirika is also highly capable in combat, taking down several gunmen on her own. Despite her deadly efficiency, Kirika admits she feels no remorse for the people she kills. She also has no memory of how she learned these skills or even her true identity.
Sensing a connection between their pasts, Mireille agrees to work with Kirika. Together, they set out to uncover the secrets of their histories. Using the code name "Noir," the pair's fragile partnership is put to the test as they take on jobs, each mission bringing them closer to discovering who they really are.