Tachibana Aoi went on a hot springs trip with her husband. Feeling a slight sense of expectation being alone for the first time in so long, she leaned in on her husband's shoulder when she heard a voice call out to them from behind: "is that... big bro and... Aoi-san...?" Her husband's younger brother, Kyouka, was coincidentally staying at the same inn. She was reunited with her brother-in-law after a long time, and that's all it should have been... But after a night of drinking, Aoi and Kyouka committed a one-night mistake...