One sunny day in her Parisian apartment, Mireille Bouquet, the hired assassin, receives an odd email from Kirika Yuumura, a Japanese student, proposing a joint venture to revisit the past. At first deeming the message nonsensical, Mireille has a change of heart once she recognizes a song from her younger years playing within the email's contents. As soon as the two convene, they are ambushed by gun-slinging assailants. Mireille withstands the attack, while Kirika remarkably obliterates several shooters independently. Despite her lethal abilities, Kirika bemoans her inability to experience any sorrow for her prey. To add to her troubles, Kirika cannot recall how she came to possess her capacities or even her genuine identity. Detecting a commonality between them, Mireille agrees to form an alliance with Kirika and together they commence their excursion to unravel the mysteries of their pasts. Going by the moniker "Noir," the twosome's flimsy connection is examined as they begin undertaking employment, each step taking them closer to the truth of their authentic selves.