Yuki Sanada, a socially anxious high school student, moves to Enoshima with his grandmother due to her job. Struggling to connect with others, Yuki’s life takes an unexpected turn when he encounters Haru, a peculiar boy who claims to be an alien. Haru, accompanied by his duck-like companion named Tapioca, insists that Yuki must help him save the world by catching a mysterious fish. Despite Yuki’s initial reluctance, he is drawn into Haru’s mission, which involves mastering the art of fishing.

The narrative introduces Akira Yamada, a skilled but aloof fisherman who becomes a reluctant mentor to Yuki and Haru. Akira’s expertise in fishing proves crucial as the trio prepares to face the "Tainted Fish," a dangerous entity threatening the balance of the world. Alongside them is Natsuki Usami, a local fishing enthusiast who initially resents Yuki but gradually becomes a supportive ally. The group’s dynamic evolves as they train together, forming bonds that help Yuki overcome his social anxiety.

As the story progresses, it is revealed that Haru’s mission is tied to a larger cosmic conflict involving his brother, Erika, who seeks to disrupt Earth’s harmony. The climax centers on a high-stakes fishing competition, where the group must work together to confront the Tainted Fish and prevent catastrophe. The series balances its fantastical elements with themes of friendship, self-discovery, and the transformative power of human connection, culminating in a resolution that underscores the importance of collaboration and personal growth.