"Oneechan ga Kita" is a 12-episode anime series that centers on the life of 13-year-old Tomoya Mizuhara, a middle school student whose quiet and ordinary life is disrupted when his 17-year-old stepsister, Ichika Mizuhara, moves in with him and his father. Ichika, who is cheerful, energetic, and overly affectionate, quickly becomes a central figure in Tomoya's daily life. Her intense attachment to him often manifests in playful teasing, physical affection, and a desire to spend as much time together as possible, which Tomoya finds both embarrassing and overwhelming.

The series explores the dynamic between Tomoya and Ichika as they navigate their new sibling relationship. Ichika's exuberant personality contrasts sharply with Tomoya's reserved and somewhat awkward demeanor, creating a comedic yet heartwarming dynamic. Despite her overbearing nature, Ichika genuinely cares for Tomoya and often goes out of her way to support him, whether it’s helping him with schoolwork or encouraging him in his hobbies. Tomoya, while frequently exasperated by Ichika's antics, gradually grows to appreciate her presence and the bond they share.

The narrative is episodic, with each episode focusing on a different slice-of-life scenario, such as Ichika trying to cook for Tomoya, accompanying him to school events, or attempting to bond with his friends. The series also introduces secondary characters, including Tomoya's classmates and Ichika's friends, who add variety to the interactions and further highlight Ichika's larger-than-life personality. While the tone is predominantly lighthearted and comedic, there are moments of sincerity that underscore the growing familial connection between the two siblings.

"Oneechan ga Kita" balances humor with themes of family and adaptation, presenting a straightforward yet engaging portrayal of sibling relationships. The series avoids delving into overly complex plotlines, instead focusing on the everyday experiences and small, relatable moments that define Tomoya and Ichika's lives together.