"Itsumo Kokoro ni Taiyō o!" centers on the life of Taiyō Ōbayashi, a cheerful and optimistic high school girl whose name, meaning "sun," reflects her radiant personality. The story begins with Taiyō moving to a new town and enrolling in a new school, where she quickly becomes a source of positivity for those around her. Her unwavering kindness and determination to help others often lead her into various challenges, both personal and interpersonal.

The narrative explores Taiyō’s relationships with her classmates, particularly her growing bond with the reserved and introverted Aoi Minase. Aoi, who initially struggles with self-doubt and social anxiety, finds herself drawn to Taiyō’s infectious energy. Through their friendship, Aoi begins to open up and confront her insecurities, while Taiyō learns to balance her desire to help others with her own emotional needs.

As the series progresses, Taiyō becomes involved in the lives of several other students, each dealing with their own struggles. These include Haruto Kurosawa, a talented but perfectionist athlete who fears failure, and Yuna Tachibana, a former child star grappling with the pressures of maintaining her public image. Taiyō’s presence acts as a catalyst for their growth, encouraging them to face their fears and embrace their true selves.

The plot is driven by episodic arcs that focus on individual characters, with Taiyō often at the center of these developments. Her interactions highlight themes of friendship, self-acceptance, and the importance of emotional support. While the tone is generally uplifting, the series does not shy away from addressing deeper issues such as loneliness, societal expectations, and the complexities of human relationships.

The story culminates in a school festival arc, where the characters come together to organize an event that symbolizes their collective growth. Taiyō’s unwavering spirit inspires her peers to overcome their challenges, reinforcing the central message of the series: the power of kindness and the importance of carrying a "sun" in one’s heart, even during difficult times. The ending leaves the characters’ futures open-ended, suggesting that their journeys of self-discovery and mutual support will continue beyond the confines of the narrative.