After her grandfather's funeral, young Rui Tenhara finds herself effectively alone. Her mother is deceased and her father is working overseas, leaving the composed girl to steel herself for a solitary life. However, a sudden call from her father changes everything: he has remarried from abroad, instantly making an office lady named Chiharu Koda her new older stepsister. Rather than let Rui live by herself, the warm-hearted and slightly boisterous Chiharu decides to move in with her, determined to build a family bond from scratch.

The manga focuses on the gentle, often humorous daily life of this newly formed family. Rui is the calm and pragmatic one, possessing an old-souled maturity that contrasts sharply with Chiharu's energetic and doting nature. A recurring source of comedy is Chiharu's notoriously terrible cooking, which Rui bluntly critiques. Despite the significant age gap and their different personalities, Chiharu embraces her role as a cool big sister, slowly melting the ice around Rui's guarded heart.

As the story progresses, their quiet world expands to include friends and classmates. Rui's social circle at school introduces a competitive dynamic, particularly with the sensitive and childish Kurumi and her sitcom-style rival, Kirika. Their back-and-forth, stemming from a past incident, adds a lively layer to the otherwise relaxed atmosphere. The narrative focuses on small, meaningful moments of togetherness, from navigating school life to simply sharing a meal, as the two stepsisters learn what it means to be a family under one roof.