The heroic dolphin helps capture a group of escaped convicts.

When Sandy Ricks learns that he is to be deprived of his pet dolphin, Flipper, he takes a rowboat and heads for a deserted island in the Florida Keys. Flipper tows the boat to the island after Sandy collapses from exhaustion. Nearby, three escaped convicts take over Sir Halsey Hopewell's yacht, keeping Hopewell aboard but sending his wife and daughters, Penny and Gwen, to the island where Sandy is hiding. Sandy remains under cover but arranges for Flipper to toss things the Hopewells will need for survival--fish, matches, knives, flashlight, and the like--up on the beach. Meanwhile, Sandy's father has begun a search for him. Sandy reveals himself to and befriends Penny but swears her to secrecy about his presence on the island. When the convicts learn that Hopewell is an influential millionaire, they return to the island and threaten to kill his family unless he arranges for their pardon. Sandy and Flipper battle the convicts and capture two of them. The third, just before he is captured, drives a knife into Flipper during an underwater fight. Hopewell radios the authorities, and the injured Flipper is flown to Miami. There he undergoes surgery, and Sandy and his father are reunited. Flipper recovers and Mr. Ricks tells Sandy that he will be able to keep his dolphin.