Somewhere traveling across the sea aboard a boat known as The Floating Palace, Sofia is in the library reading a story when the royal family arrives at a tropical island known as Merroway Cove for a vacation. Also accompanying the Royal family is Cedric who is out on another scheme to take over the kingdom only this time he's not after the Amulet of Avalor. On the swimming deck Sofia spots a young mermaid who gets herself tangled in a fishing net. Sofia helps the young mermaid also known as Oona get untangled only to have a hurt fin. Then to their amazement, Sofia's amulet grants her the power to turn into a mermaid. Happy with her new powers, Sofia decides to head down below and help Oona return home.