Nineteen-year-old Setoya Kinari leaves her rural hometown to attend university in Tokyo, her first time living away from home. Her two goals are to move into the lively share house called Kitten Blue and to find a new love in the big city. On her first day, she is mistaken for a stalker by one of the residents, the strikingly handsome but aloof college sophomore Kashiwagi Towa. After the misunderstanding is cleared up, Towa warms up to her, though he remains teasing and physically close in a way that keeps Kinari off balance. Despite his flirtatious reputation, she sees his caring side when he helps her adjust to city life, shows her the moon through a telescope, and quietly supports her.

Kinari tries to resist falling for him, telling herself he is too dangerous a person to love, but Towa keeps drawing her in. The share house is filled with other colourful residents: the gentle and cooky social worker Wataru Yoshise, the cheerful dancer Mizuki, and the flashy hostess Machiko. They all form a makeshift family. As Kinari and Towa grow closer, they enter a phase of mutual pining. Towa confesses his feelings, and they begin dating. Their relationship deepens, but complications arise when Kinari’s first love, Ichiya, reappears and moves into the share house, triggering jealousy and a revisit of old feelings. Eventually Ichiya leaves, and Kinari and Towa solidify their bond.

Later arcs see the couple navigate life as a secret couple in the share house after Wataru’s marriage leads to a new rule forbidding public displays of affection. Towa’s possessive, “S-dele” personality contrasts with his sweet side. They take a trip to Okinawa for Wataru’s wedding, share a hotel room, and face new tensions. A new male high school student, Sui, moves into Kitten Blue and develops a crush on Kinari, creating further jealousy. As Towa graduates and begins working, the couple decide to leave the share house and move in together, starting a cohabitation arc that tests their ability to balance his busy career with their relationship. Throughout the series, the story focuses on the challenges and joys of growing up, trusting a partner, and building a life under one roof.