There is an underground trading system called "Spiritual Body Contract" hidden in modern cities. The contractor (the protagonist) has a special physique and allows the undead to borrow his body for a period of time. Each transaction will bring certain dangers to the contractor, because during this period of time, the contractor's soul will leave the body, and he must beware of being snatched away by the living soul at any time, and the undead needs to pay a "non-material price" - which may be obsessions, emotions or special abilities during his lifetime.
