Suzuna Aoi is a woman with terrible luck in love. Her parents are away on business trips, and her younger brother, Ritsu, has left the house to attend college. Suzuna has been living alone in their family home, but one day, Ritsu suddenly returns. Ritsu is her stepbrother from her parents' remarriage, and he announces that he will be "living here again" now that he has gotten a job. Suzuna is flustered by Ritsu's return as a "grown-up man"...! He's her brother, but not really her brother-- a fragile and precarious love story between a stepsister and stepbrother!