"Ōkii Ichinensei to Chiisana Ninensei" centers on the unlikely friendship between Tatsuya, a tall and gentle first-grader, and Ayaka, a small but spirited second-grader. Despite their age and size differences, the two form a close bond as they navigate the everyday challenges and joys of elementary school life. Tatsuya, often mistaken for being older due to his height, is kind-hearted and thoughtful, while Ayaka, though younger, is confident and assertive, often taking the lead in their interactions. Their dynamic is characterized by mutual respect and a shared sense of curiosity about the world around them.

The series unfolds through a series of episodic vignettes, each highlighting small but meaningful moments in their daily lives. These include classroom activities, interactions with classmates, and adventures during recess. The narrative emphasizes themes of friendship, empathy, and the innocence of childhood, portraying how Tatsuya and Ayaka learn from each other and grow together. Supporting characters, such as their classmates and teachers, add depth to the story, showcasing the diverse personalities and experiences within their school environment.

The tone of the series is warm and lighthearted, with a focus on the simple yet profound experiences of childhood. Through its slice-of-life approach, the anime captures the essence of growing up, highlighting the importance of understanding and supporting one another. The relationship between Tatsuya and Ayaka serves as the emotional core of the story, illustrating how even the smallest connections can have a significant impact.