Sagara Yuichi is an aspiring writer who was caught in a convenience store robbery. Despite having the chance to escape, he lingered to gain writer's experience, and ended up getting stabbed. During his last breath, a strange guy appeared claiming it to be fate. A few hours later, he awakens and finds himself sleeping naked next to said strange guy. Then, the latter tells him that his blood is "S-Class" and that he's going to be Yuichi's lifetime partner. Will the pragmatic novelist be able to wrap his mind around all this…?!