Yuri lives with his elderly astronomer grandfather in a small observatory on a sea-surrounded island. Their days blend fishing with stargazing, Yuri skilled with a hand-thrown harpoon.

One evening, using an unusual telescope, Yuri discovers an unsettling anomaly: the Ursa Minor constellation has gained an extra star. Alarm grows as he watches the constellation warp, its bear shape twisting into a monstrous fish. This entity begins consuming stars, wreaking havoc across the Milky Way, envisioned as a vast cosmic ocean.

Setting out in their small rowboat, Yuri and his grandfather sail this starry ocean. They traverse landscapes merging land and sky, encountering a talking forest, sentient buildings lamenting lost stars, and mythical flying beasts. Their voyage leads them to giants forging stars and a wizard. The wizard bestows upon Yuri a magic harpoon, charging him to confront the cosmic fish.

Yuri proceeds with calm determination, observantly appreciating the world, including glimpses of undersea life. The journey culminates when Yuri successfully uses the magic harpoon against the star-devouring entity. Afterwards, quiet serenity returns. The tale concludes simply with Yuri and his grandfather resuming their routine commute, a sense of duty fulfilled and peaceful closure settling over them.