"Moeru! Oniisan" centers on the life of Kazuma Onimaru, a 25-year-old man who unexpectedly becomes the guardian of his five younger stepsiblings after their parents pass away. The siblings—Yayoi, Hazuki, Satsuki, Uzuki, and Kisaragi—range in age from elementary school to high school, each with distinct personalities and challenges. Kazuma, a former delinquent with a tough exterior, struggles to adapt to his new role as a caregiver while balancing his own life and responsibilities. The series explores the dynamics of their unconventional family as Kazuma learns to navigate the complexities of parenting, sibling relationships, and personal growth.

The narrative unfolds through a mix of comedic and heartfelt moments, highlighting the siblings' daily lives and the challenges they face together. Yayoi, the eldest sister, often takes on a maternal role, while Hazuki, the second eldest, is more reserved and studious. Satsuki, the middle child, is energetic and mischievous, often causing trouble. Uzuki, the youngest sister, is shy and artistic, while Kisaragi, the only brother, is quiet and introspective. Kazuma’s journey involves overcoming his own insecurities and past mistakes to provide stability and support for his siblings, who in turn help him mature and find purpose.

The series delves into themes of family, responsibility, and resilience, portraying the struggles and triumphs of a makeshift family striving to stay united. Through various episodic scenarios, the siblings and Kazuma grow closer, learning to rely on one another despite their differences. The story balances lighthearted humor with poignant moments, creating a narrative that emphasizes the importance of love, understanding, and perseverance in the face of adversity.