On vacation in Australia, Sabrina joins up with Gwen, the British witch she met on her last vacation in Rome. Together they explore the wonders of the Great Barrier Reef, go on a deep-sea dive with famous marine biologist Dr. Julian Martin, and meet the mysterious and very cute Barnaby...who turns out to be a merman! However, something's fishy when a strange sea virus threatens the local aquatic population. Sabrina and Gwen vow to restore the ocean to its original glory, especially since the disease is affecting their new friend Barnaby. The young witches must find a way to help the doomed sea creatures and at the same time, protect the secrecy of the mer-species...