Set in the near future, Mireille is a young woman, a lone-wolf type who just happens to be one of the most talented assassins in the underworld. One day she receives an email with the name of her next target: Yuumura Kirika. The email has an attachment phrase, "Join me on a pilgrimage to the past." and a melody that was playing from a music box when Mireille's parents were killed by a mysterious assassin. Mireille and Kirika, a girl who has no memory of her past, eventually team up as assassins.