After having found a magic locket which shows the kingdom of Atlantica, Melody decides to run away from home and find the truth behind it. Ariel eventually discovers that her daughter has run away after being told by Sebastian. Ariel must turn back into a mermaid to go into the sea once more to find her missing daughter. However, Ursula's crazy sister, Morgana, intends to take control over the entire ocean. Ariel and her friends must stop Morgana from accomplishing the mission that Ursula failed and save Melody from her evil clutches.To protect her from a sea witch, Ariel's daughter is not allowed in the ocean; but when she becomes 12, she runs away to an adventure under the sea.