Aki's family is cursed, specifically on his mother's side. Unfortunately, the curse isn't limited to his mother's side. His father, who married in, suffered from the same affliction and ended up with the same fate as Aki's mother and many other family members over the years: death. The only one left is Aki's grandfather, who is frail and sick, just like all of his family members who passed before him. In his final moments, Aki's grandfather tells Aki about someone who can help him survive the curse but doesn't provide much more information before he passes.
On his own, Aki is doing his best to get through life but grows frail as he is struck with an incurable disease he attributes to the curse. After passing out and being unable to return to work, Aki is desperate to keep himself alive and beat the curse. As he is on the cusp of succumbing to the curse, he follows his grandfather's advice and finds a bone hidden away. Disheartened by the useless discovery, Aki is ready to accept his fate when he gets blood on the bone, causing it to grow and expand with more bones, flesh, muscles, and skin, until it takes the form of a half-man, half-god.
As it turns out, Aki's family acquired a guardian deity. The reason Aki's grandfather was able to live to old age is that this guardian deity helped consume the curse, prolonging his life. Aki can hardly believe it until the deity reaches into his body and pulls out a part of the curse, providing instant relief. Unfortunately, the best way for this deity to find the curse is to be intimate with the person. Now that Aki has completed the contract by giving the deity a name, Setsu, he has to suffer through the uncomfortable touch to keep himself alive. Even as he continues to claim he dislikes it all, he finds himself looking forward to the pleasure he can receive from Setsu. Once Setsu isn't needed anymore, he'll turn back into bone. Once the curse is cured, can Aki return to a life without Setsu?