Hyeseong, a boy living on a remote island, and Yunhae, a mysterious mermaid, become each other's only friend. But soon after they meet, they're separated after Hyeseong moves away. Six years later, Yunhae develops a mysterious power and finds Hyeseong again. Unfortunately, too much has changed, and Hyeseong can no longer dedicate all his time to her as he did in their childhood. He tells her they are too different, but Yunhae's attachment to Hyeseong slowly transforms into an obsession...