This is a two part episode. Porter and Sandy are battening down the launch for a tropical storm and learn after it passes from Flipper that a man was caught in the storm in a skiff. This man turns out to be a bank robber who has buried $10,000's in cash in sand on a beach and feigns injury when Porter and Sandy show up. They take him to their home where he calls his brother (who is complicit in the crime) to come and get him. All seems innocent to Porter until he gets an RT call to be on the lookout for the criminals prompting the robber to pull a gun on Porter and Sandy.
Bud returns from fishing and is able to send Flipper for help before being forced into his home to sit and wait with Porter and Sandy for the robber's brother to come down from Miami. Flipper tries to attract the attention of boat owners, one who calls in to Porter and Porter is forced at gun point to say nothing. Finally Flipper interferes with a fish haul that causes an old fisherman to lose his catch. Angered he and his son head to Coral Key to tell Porter and they arrive just as the robber returns from retrieving the cash from the beach enabling Porter to tackle him into the water and the fisherman's son to do the same to the fleeing brother.
Small detail point - the brother is coming from Miami in a reasonably fast boat that Porter estimates will take 3 hours to get to Coral Key implying that the fictional Coral Key is right at the end of the Keys when in reality in several episodes the skyline of Miami is seen in the distance because the show was shot at Key Biscayne just south of Miami.