"Yo, Aquarist" is a story that begins with a frustrated delinquent boy picking up an abandoned goldfish on the night of a festival. The main character, who had an experience when he tried to buy a goldfish at home when he was a child and let it die, tries to clear his regret that he should have escaped to the river instead of trying to keep it. However, it is forbidden to throw away goldfish in the river, and a mysterious girl calls out to him.