One fine day Ariel, Flounder and Sebastian are relaxing on the ocean surface. (Sebastian relaxing? It has to be seen to be believed.) Just as they're lounging about, an ominous fin appears in the water... But never fear, it's only Spot, the adorable killer whale that was Ariel's temporary pet some time ago. It seems that he's on his yearly migration route and has decided to drop by. The reunion of the gang is cheerful, even if Flounder is a little alarmed by how much Spot has grown.
For some reason or other Ariel decides that she wants the Atlanticans to get to know Spot. He does, after all, have a flair for performing and Ariel thinks that when the Atlanticans see how tame and lovable he is, they'd stop being nervous around him. Triton approves of this idea, and they set up a circus-like performance for the Atlanticans by Spot.
As Spot is having fun splashing in and out of the water, he is spotted by humans on a ship. The owner is Pettigrew, a man who owns a Penguin Park. Pettigrew decides that a killer whale would be an excellent addition to his park and decides to catch Spot despite his son's protests. The ship sails closer and drops a large net around Spot, and underwater the Atlanticans disperse in a panic.
Ariel, of course, is not going to take it lying down, even if humans are involved. She goes off to rescue Spot, with the aid of Sebastian and Flounder. They follow the boat to Pettigrew's Penguin Park and swim in through the large doors just as it's closing. There, they see Pettigrew bully his penguins into performing. Sebastian too has been caught and is "trained" as a performing crustacean.
Later that night Ariel sets her plan into motion. Suffice to say that some things go right and some things go wrong, but in the end it's Pettigrew's heart-of-gold son that helps Spot and the penguins escape.