Circa 1900, in Ireland, two carnies, Angus Shaw (Rufus Sewell) and his wife Lillian (Carla Gugino), abduct a mermaid (Rya Kihlstedt) and try to smuggle her into the United States. As their ship appears to lose its way, the mermaid begins to show a deadly, carnivorous side.
Peter (Levi Miller) is a young boy who is left as a baby on the steps of an orphanage in London by his mother Mary (Amanda Seyfried), an establishment under the care of Mother Barnabas (Kathy Burke). Several years later, during World War II,The Cold War, upon learning that she is hoarding food for herself, Peter and his best friend Nibs try to steal it to distribute amongst themselves and the other orphans but they get caught. In the process, Peter finds a letter written by his mother, declaring her love and assuring Peter they will meet again "in this world or another".
The daughter of a man who owns a South Seas pearl business falls in love with a wealthy traveler. Her father dies, leaving her the business, but a greedy ship captain schemes to take the business from her.
Ariel and Eric celebrate the birth of their newborn daughter Melody on a ship at sea. Ariel's father King Triton presents Melody with a magic locket. The party is interrupted by Morgana, sister of the deceased Ursula, who threatens to cause Melody harm, using her as leverage to get Triton's trident. Ariel and Eric work together to foil Morgana's plan.
Shishioh, a popular and successful professional wrestler, builds a large mansion for his wife Asami and their kids. During the housewarming party, Ichijo, an angry competitor, causes a riot that destroys the house. Asami is hospitalized, where she contracts a rare virus that slowly transforms her into a mermaid. Desperate for money to finance his new house and find a cure for Asami, Shishioh agrees to a reality TV show in which he wrestles several competitors in a haunted house. His opponents turn out to be homicidal killers and include a zombie among them. Eventually, Shishioh learns that the television producer infected Asami with an experimental virus in order to get him to compete.
A singer, Sugar Kane (Linda Evans), is unwittingly being used for publicity stunts for her latest album by her agent (Paul Lynde), for example, faking a skydiving stunt, actually performed by Bonnie (Deborah Walley). Meanwhile, Frankie (Frankie Avalon), (duped into thinking he rescued Sugar Kane), takes up skydiving at Bonnie's prompting; she secretly wants to make her boyfriend Steve (John Ashley) jealous. This, of course, prompts Dee Dee (Annette Funicello) to also try free-falling. Eric Von Zipper (Harvey Lembeck) and his Malibu Rat Pack bikers also show up, with Von Zipper falling madly in love with Sugar Kane. To top all this, Bonehead (Jody McCrea) falls in love with a mermaid (Marta Kristen). Eventually, Von Zipper "puts the snatch" on Sugar Kane. The film takes a The Perils of Pauline-like twist, with the evil South Dakota Slim (Timothy Carey) kidnapping Sugar and tying her to a buzz-saw.
Best friends Hailey Rogers (JoJo) and Claire Brown (Emma Roberts) are upset to know that they will be separated in five days as Hailey has to move to Australia because of her mother's (Claudia Karvan) job. Claire, senisible and slightly naive, looks up to Hailey as a mother figure in some ways, since her parents drowned in a boat accident years before. She now lives with her grandparents by the beach near Hailey, who is more daring and adventurous, in a small Florida town.
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.
William Humphries (Kip Pardue) is at the end of his trust fund after several failed career-attempts. Louis Baxter (Matthew Edison), William's friend, takes him along to the annual endangered animals benefit in L.A. This year, to raise more money, Princess Ithaca (Nora Zehetner) auctions a single dinner, which William bids the last of his money on believing that knowing the princess will open business opportunities for him. He wins the auction, but finds himself enchanted by Ithaca, who believes he is the seeker, a long-awaited champion who will help save a secret realm of mythological creatures.
Unscrupulous fisherman John Mallick and his assistant Carlo are fishing with soda bombs in open water to harvest coral reefs when they notice a large creature thrashing in the water, Mallick shoots it and discovers that it's a merman. They attempt to collect the body but the vessel is knocked by a powerful force. Mallick sees a figure in the water, pointing a trident at him an he and Carlo escape, without the body of the merman.
Sudsakorn, the son of a mermaid and a minstrel prince, fights on different occasions, an elephant, shark, and dragon horse, and encounters in his meanderings a king, a hermit, a yogi, a magic wand, and ghosts.
Lena is a princess mermaid, sea king's daughter who dreams of someday marrying a wonderful human prince. After disobeying her father, Lena visits the sea surface (after being yanked by a whirlpool) and first sees the Prince Stephan, falling madly in love. Soon after, the ship that sails the Prince Stephan sinks and the youth's life in danger until Lena manages to save him and bring him to a beach near the school where he studied. Lena comes to the aid of the witch Cassandra and exchanges her voice for legs to walk on land and meet her prince. Cassandra gives Lena what she asks for with the warning that if Lena fails to win the heart of Stephan, she will become sea foam for the rest of her life. Lena drinks the potion, loses her voice, and goes to the surface. Above, she walks on shore naked (except for her shawl, flower necklace and the wreath of pearls her sister Tris gave her for her birthday). She is later helped at the palace by two doves, Cecil and Claudia. Things are going well, but Lena is dumb. Prince Stephan takes a fondness to her, as he feels he should show her the same kindness that the person who had saved him from the wreck showed him. Over time, Stephan falls in love with Lena and wants to marry her, but then comes Anna, the young princess who earlier found Stephan and Cosgrove on the beach and brought them to the medical care they needed. Stephan agrees to marry Anna, but Anna is in love with Maxwell. Vink, the dolphin pet friend of Lena, intervenes and tells the king of the sea what happened. His intervention fixes things, King Malcolm agrees to make Maxwell his son-in-law, and Lena finally marries Stephan, saving her life.
Follows three former National Oceanic Atmospheric Administration scientists and an ex-Government employee, after investigating mass stranding of whales, claim to have recorded mysterious underwater noises coming from an unknown source. This sound resembled a sound previously recorded in 1997, called the "bloop". They recovered 30% of the remains of an unknown creature from inside a great white shark which was said to possess attributes of the human body. They alleged that the marine creature had hands, not fins, and the hip structure of an upright animal. These findings, along with many others led the team to determine that this unknown animal was very closely related to humans — a mermaid.
Widowed Classics Professor Thomas Bradley (Billy Zane) is about to lose his research grant studying ancient love spells at Oxford University. He knows he is close to a major academic breakthrough and desperately needs more time to complete his research. Thomas is given one last chance to finish his work, and goes to the Greek island of Spetses, where he has been invited to stay at the home of a supportive Oxford don Professor Coulter. Thomas brings along his 12-year-old daughter, Serena (Amber Savva). The house is in tolerable condition, and Serena finds a book about merpeople that has an ancient Greek inscription on the cover.