The pups and Capt'n Turbot must join with the Mer-creatures to save Ryder and the secret Mer-home by breaking a curse while they make an amazing discovery.
11 years ago, Capt'n Turbot, who was a teen named Horatio Turbot back then, was best friends with 16 year old Zoey Ryder who loved the ocean even more than he does and enjoys long walks on the beach with him and hanging out with a walrus calf (aka: Wally). One day, the Ryders moved away from Adventure Bay and the ocean which made Zoey very unhappy. When the Ryders came to their new home, her parents announced that they were going to have a baby (Ryder). Zoey got angry and believed they moved just to have a new baby. Later that night, she ran away and took a bus back to Adventure Bay and met up with Turbot on the beach. While there, they discovered an oyster with a magic purple pearl that could talk. She told them that she was Lavender the Purple Dolphin Fairy until an evil sea-warlock put a curse on her. She forgot to warn Zoey to put her back in the oyster too late. If anyone holds her outside of her oyster for over a minute, an unbreakable spell will be casted on them. Suddenly, Zoey was surrounded in a purple aura and disappeared. All Turbot found left of her was her shell bracelet.
In the present day, the pups find Turbot's secret basement which contains of picture of him and Zoey, a shell bracelet, and Lavender's oyster. Turbot finds them in the basement and sees Lavender. She tells them that she sees the evil sea-warlock is plotting terrible things. When they leave the lighthouse, they see Ryder get dragged into the sea by a dark aura. Suddenly, the Mer-Pups and a Mermaid appear in front of them. Turbot recognizes the Mermaid is really Zoey. Turbot gets his Diving Bell while the Mer-Pups turn the pups into Mer-Pups and they dive down with them to rescue Ryder and stop the sea-warlock. Will they stop the warlock and break the curse?