In the heart of Paris, Mireille Bouquet, a professional assassin, receives an enigmatic email from Kirika Yuumura, a mysterious Japanese student, beckoning her to revisit the bygone days. Intrigued by a nostalgic tune in the message, Mireille's skepticism wanes. Their first face-to-face encounter is marred by a sudden attack, revealing Kirika's surprising combat prowess. Despite her skill in dispatching foes, Kirika grapples with her lack of remorse for her actions and a puzzling amnesia shrouding her past. Sensing a shared yet enigmatic connection, Mireille joins forces with Kirika as they delve into the shadows of their murky histories. Adopting the alias 'Noir,' the duo confronts escalating trials and revelations with each mission, inching closer to unraveling the secrets that bind their fates.