Ruka, a spirited teenage girl, feels misunderstood by her peers and disconnected from her family. After getting into trouble at her school's handball club, she's left with an empty summer ahead. Ruka decides to pass time at the aquarium where her father works and discovers two mysterious boys, Umi and Sora, who seem to have an extraordinary connection to the sea. Raised by dugongs, the boys possess a unique affinity for marine life that intrigues Ruka. As she spends more time with them, Ruka begins to experience strange visions and senses a hidden link between herself, her new friends, and the ocean depths.

As a series of unusual natural events unfold, Ruka learns that Umi and Sora are at the center of a cosmic phenomenon that affects all aquatic life on Earth. The trio embarks on an adventure that bridges the natural and supernatural worlds and leads Ruka to confront her inner fears and unanswered questions about her familial bonds and her purpose. As their journey progresses, the mystical connection between Ruka, Umi, Sora, and the ocean reveals deeper mysteries that challenge their understanding of life and existence.