For centuries, the deadly arts have been perfected by Masters, and Five Great Families hold the greatest power and influence. Hikaru Gero, an heir to the Gero family, lives in the underworld as an elite assassin. When his sister is pressured to produce an heir, Hikaru asks Mei Kinosaki, a brilliant marriage swindler, to find him a bride. The most challenging mission of his life begins!

The most exquisite hotels in the world have certain standards: the finest food, the most sublime comfort, captivating entertainment, the latest weapons, reliable identity fabrication, and untraceable corpse disposal. The name of this place where the people who have "forgotten their humanity" visit is "Hotel In Humans." The hitmen guests are welcomed by the concierges who never say "no."