One day, Mitsuda Takuzo is approached by his ex-girlfriend, Yanoguchi Mia, and her current boyfriend, Satomura Shinpei, who propose the idea of a "three-person marriage". Initially, Takuzo dismisses the idea as absurd, but he gradually agrees to a one-year trial period of shared living. As they navigate this unconventional arrangement, the trio faces numerous challenges, including setting boundaries for their sexual relationships, managing jealousy, and explaining their situation to their parents. Through it all, they strive to define their own version of happiness.