The class goes to the beach, and Chacha, Riiya, and Shiine decide to go to an island just offshore. Riiya falls into the ocean and Chacha knocks him out with a giant anchor while trying to rescue him. This causes him to have amnesia. A mermaid named Marin rescues Riiya as he is sinking toward the bottom, and then promptly falls in love with him. She spends the rest of the episode trying to keep Chacha away from Riiya so she can keep him all to herself.