Velma is feeling like a fifth wheel. Fred has Daphne, and Shaggy has Scooby. Who does she have? So, when a mysterious "Mer-woman" named Amy contacts Velma on her blog, she decides that maybe this will be her special friend. Amy tells her that "Fish Freaks" have been attacking boats in Crystal Cove harbor. When Velma can't convince the gang to help her, she investigates alone - and almost gets herself killed! Without revealing her source (because she doesn't want anyone stealing her new friend), Velma tells the gang about the Fish Freaks and they begin to investigate the mystery. Turns out, the Fish Freaks are guarding a secret on a haunted oil derrick out in the ocean. But the closer the gang gets to solving the mystery, the more dangerous their investigation becomes - and the more Velma risks losing her special friend.