Sabrina travels to Australia's Great Barrier Reef with Gwen, a fellow witch from England, for a week-long vacation where they try to help protect a hidden mermaid colony whose habitat is threatened by ocean pollution, and by local marine biologist Dr. Julian Martin, who is determined to find the colony as his claim to fame. While Sabrina finds a friendship with Barnaby, a "merman" from the mermaid colony, Salem the cat finds a possible romance with another witch-turned-into-a-cat named Hilary, but finds Sabrina's problems interfering with his plans. In order to prevent Julian from launching a search for the mermaid colony, she casts a spell to create a storm to keep him in port, which succeeds except that a lightning bolt knocks Sabrina unconscious, and when she comes to she learns she has lost her powers, at least temporarily.
- Melissa Joan Hart as Sabrina Spellman
- Tara Strong as Gwen
- Scott Michaelson as Barnaby
- Lindsay Sloane as Fin
- Nick Bakay as the voice of Salem Saberhagen
- Peter O'Brien as Dr. Julian Martin
- Rebecca Gibney as Hilary Hexton
- Conrad Coleby as Jerome
- Ben Lawson has a small appearance as an employee of the reside.