Sharks cause trouble in Trident City.

Mermaid Echo is doing tail flips because she's going to see her favorite under-the-sea band, the Rays. They're performing for Pearl's birthday party and everyone at Trident Academy is invited except Echo's best friend Shelly.

Unable to comprehend a beloved librarian's imminent marriage to a cafeteria worker who seems to be perpetually grouchy, Pearl becomes jealous when Echo, Shelly, and Kiki are all asked to be flower girls.

Mermaids Shelly and Echo are excited to begin third grade at the prestigious Trident Academy, but snooty Pearl, jokester Rocky, brilliant Kiki, grumpy Mr. Fangtooth, and an argument over their first project challenge the best friends. Includes facts about marine plants and animals and words to the Mermaid Tales Song.

Echo's third grade class visit the Titanic and figure out a way of including their new classmate, Anita, who is confined to a wheeled chair.

Shelly Siren and Trident Academy's Shell Wars team travel to the city of Atlantis for the championship game. The mergirls get a tour of Atlantis, and learn about how the city ended up at the bottom of the ocean. But when they swim through Poseidon's Temple, Shelly spots a spookfish, which legend has it, carries a horrible curse.