Mariko is divorced and lives with her eldest son Naoya and younger son Daisuke. Due to the absence of a father figure, the bond between the brothers is weak, and they constantly fight. Mariko repeatedly tells them to "share things," but in reality, she has been having a physical relationship exclusively with Naoya. One day, Daisuke discovers the truth. Forced to make a decision, Mariko offers her other hole—her anus—to Daisuke, proposing that they "share" her. A shocking story of transformed brotherly bonds begins.