Hakofugu (Black spotted boxfish) lacks confidence and always stays alone by the rocks, watching other fish and feeling envious. Then, one day, upon meeting Himetatsu (Korean seahorse), who has lost the way and is said to be from the sea of Minamata, Boxfish's very first adventure begins. This film is based on the picture book "The Boxfish's Wish" (published by Kodansha), written by Sakana-kun (Fish Guy) — a Japanese marine biologist, TV personality, and illustrator.