"Nozomi in the Sun" centers on Nozomi Aihara, a high school student living in a small coastal town. Her life takes an unexpected turn when she discovers a mysterious seashell that grants her the ability to communicate with the ocean. This newfound connection reveals that the ocean is a sentient entity facing a dire threat from pollution and environmental neglect. Nozomi, alongside her childhood friend Renji Takeda and a transfer student named Haruka Miyamoto, embarks on a mission to protect the ocean and uncover the truth behind its distress.

The narrative unfolds as Nozomi learns to harness her abilities, which grow stronger as she deepens her bond with the ocean. The trio encounters various challenges, including skeptical townsfolk, corporate interests exploiting marine resources, and supernatural forces tied to the ocean's decline. Each episode delves into themes of environmentalism, community, and personal growth, as Nozomi and her friends work to balance their responsibilities as students with their mission to save the ocean.

Key characters include Nozomi, whose determination and empathy drive the story; Renji, a pragmatic and resourceful individual who provides emotional support and practical solutions; and Haruka, whose scientific curiosity and analytical mind complement Nozomi's intuitive approach. The series also features antagonists such as Mr. Kuroda, a corporate executive whose actions exacerbate the ocean's plight, and Kaida, a mysterious figure with a complex connection to the ocean's sentience.

The plot progresses through a series of episodic challenges and overarching mysteries, culminating in a climactic confrontation that tests Nozomi's resolve and the strength of her friendships. The story concludes with a resolution that emphasizes the importance of environmental stewardship and the power of collective action, leaving a lasting impact on the characters and their community.