Cody Griffin is an adopted teenager. His biological mother is a mermaid who left him on the Griffins' boat when he was a baby. As she left, she was spotted by Mr. Wheatly, a fisherman who had since became obsessed with finding mermaids. Cody establishes himself as a quick swimmer in the town he lives in, attracting the attention of a girl named Sam. He's under a lot pressure from a big swim meet coming up and because he's failing biology. He's partnered with Jess Wheatly, the class geek who is a marine biology expert.
After Cody's thirteenth birthday, he begins to feel strange. As he wakes up in the morning, he turns off his alarm clock and zaps it. Thinking nothing of it, Cody goes downstairs and drinks some milk, which is when he notices that something is wrong as his hand sticks to the milk container. He goes to Jess for help, who agrees to figure out what's going on with him as long as Cody teaches him to swim in return.
Jess runs various tests on Cody and learns that Cody can generate electricity, climb walls, talk to fish, swim extremely fast, and sometimes, when he gets wet, scales appear on his hands and arms. Eventually, he comes to the conclusion that Cody is turning into a merman.
In spite of the strange reactions of his body to water, as well as the objection of his adoptive parents when they found out, Cody decides to attend the swim meet. He not only wins the meet and beats Sean, but breaks the state speed record, sprouting fins in the process in addition to the scales. Sean sees the scales and surmised that Cody was cheating. He follows him to the locker room to confront him. Cody avoids him by sticking on to the ceiling, but Sean calls out to him, saying that he saw something and will find out what. Cody safely makes it out of the stadium and goes home with his parents after they find out that he was not in his room. The scales on his arms aren't going away. Cody tries to calm his parents by claiming that nobody saw the changes, but Jess shows up later and tells them that he saw everything and that people are talking about Cody.
Sam shows up at Cody's house and faints at the sight of his scales. When she comes to, she is freaked out. Cody tells her that he's still the same guy and to not be afraid. While sworn to secrecy for Cody's safety, Sam is disturbed by the discovery and leaves.
Sometime later, Cody finds Sam at the beach. He shows her his regular arms, and tells her he wants to show her something. They go into the water and after a moment, Cody is able to contact his mermaid mother. Cody suddenly gasps in pain and stumbles back to the shore. While lying on the ground, he tells Sam to go get his parents.
After she leaves, Cody's legs start to turn into a tail. Thinking that Sam has come back, Cody is surprised when someone throws a blanket over him. Jess comes down to where Cody is, but sees that he is gone. He then sees his father's boat and panics because his father had seen Cody's fins at the swim meet earlier. Jess's father is planning on using Cody to lure in the mermaid. Jess comes on board his father's ship, and Cody asks him for help. Just then, his mermaid mother appears. He and Jess try to warn the mermaid, but Jess's father drops a fishing net on her and traps her in it.
Cody asks Jess to help his mother, so he takes a knife and jumps into the water to cut the net. He frees the mermaid, but the net gets caught on his leg, pulling him under. Jess passes out, but Cody jumps in and brings him to the dock where Jess's father, Cody's parents, and Sam are. Once Jess is pulled out of the water, Mrs. Griffin and Sam perform CPR on him, but Jess doesn't respond to it. Cody then decides to use his electricity to shock Jess's heart, which successfully makes Jess come to.
Cody's mermaid mother shows up again and Cody tells his parents that she wants him to go with her and that he has to because she's the only one that can help him with his changes. Cody's adoptive mother doesn't want him to leave at first, but she comes to an agreement with the mermaid who telepathically promises to let Cody back before school starts.
After saying goodbye, Jess tells Cody that he and his dad want to know about everything that happens while he's gone and Sam tells him not to fall for any mergirls. Denuded himself, Cody dives into the water and completes his transformation into a merman with his mermaid mother's help. He swims back up and leaps through the surface, showing his fully transformed fish tail, and waves good bye to his friends and adoptive parents before swimming off.
- Chez Starbuck - Cody Griffin
- Bridget & Kendra Byrd - Cody as a baby
- Justin Jon Ross - Jess Wheatley
- Courtnee Draper - Sam
- Brent Briscoe - Big John Wheatley
- Tim Redwine - Sean Marshall
- Dave Coulier - Whit Griffin
- Lisa Stahl - Sharon Griffin (as Lisa Stahl Sullivan)
- Brian Haley - Coach
- Kristen Stewart - Extra - In the Fountain Line (uncredited)
- HBHS Marching Band - Background Band
- Stephanie Chantel Durelli - Mermaid