Hiro Hiyorimi is a boy who has just moved to the city to meet his sister. While walking down the sidewalk, he is suddenly hit by a car (crushed by construction beams in the anime). On the verge of death, he is seen by a young woman clad in gothic clothes. She then resurrects him, leaving him alive in the hospital morgue. He wakes up confused over what happened, only to encounter the woman again as she eliminates some wolf creatures. She addresses Hiro as "her servant" and introduces herself as "Hime" (Japanese for Princess) and explains that she is a member of the royal family of the Monster Realm.
All members of the royal family are endowed with the power to turn a dead body into an immortal warrior that will serve and protect them; this means that a corpse that drinks her blood (or to she bestows the flame of immortality in the anime) shall become semi-immortal; this is how Hiro came back to life. They can't be killed, even if their heads are severed; it can be reattached, and appendages can eventually regrow. The "semi" means that if Hiro doesn't drink more of Hime's blood (or she replenishes the flame in the anime) every few days, he will die permanently
Hiro is now Hime's immortal warrior, but she herself isn't immortal yet (which is why she needs help from warriors). The story then follows Hiro as he assists Hime in killing creatures that have been sent by her siblings to attack her in the competition for the Monster Kingdom's throne.