This OVA is based on a fairy tale written by Ryunosuke Takeshita before he entered elementary school (at the time). The original author also supervised the production. Ryunosuke's younger sister, Eri-chan, is only 11 months old, but she can accurately predict the weather through the curls of her hair, earning her the nickname "Genius Eri-chan." One day, Eri-chan accidentally swallows a goldfish bought at a festival. Moreover, the goldfish is swimming happily in the pool of milk inside her belly...