A boxfish too afraid to leave the rocks meets a lost seahorse, and suddenly his entire life of watching from the sidelines stops being enough. The journey to return one small fish to a distant sea becomes the permission slip he never knew he needed to become someone other than afraid. By the end, he's not rescuing the seahorse, he's rescuing himself.

A lonely boxfish named Hakofugu spends his days alone on the same rocks, watching the other fish and envying them. Unsure of himself and stuck in routine, he convinces himself that everyone around him is luckier than he is. One day he meets Himetatsu, a lost young seahorse who appears to have come from the Sea of Minamata. For the first time, the boxfish decides to set out on an adventure of his own: returning the seahorse to Minamata's waters. Adapted from a picture book by Sakana-kun, this fantasy adventure film follows the two as they travel together toward the Sea of Minamata.