The Blair twins vacation with Mr. Renshaw and his wife, Alicia on a remote island off the coast of Florida. Earlier, the twins heard police are looking for two bank robbers
who are carrying a brown satchel filled with the stolen money. Connie, Kit, and Sterling Witherspoon, who is carrying a brown leather valise travel to the island by boat where they meet guides from the island, Rusty Longbow and Chip Wight. While Connie and Kit are on the boat they hear a blood curdling scream coming from their island destination. Chip tells the twins, offhandedly that the scream came from Kulu, the 'pet' chimpanzee belonging to the hotel manager, Mr. Talmadge. When the boat docks Rusty tries to carry Sterling's valise, but Sterling unkindly tells him to leave it alone.

While the guests are dining, Mr. Talmadge announces that Kulu has escaped. Connie goes off shelling alone, discovers a huge shell pile in a hidden cove, and is knocked unconscious. She regains consciousness hours later to find herself in another cove. Rusty finds her in the morning and takes her back to the hotel where she learns that Kulu has returned.

Musicians from the mainland entertain the guests that evening. Alicia hears about the hidden cove and says she's going shelling there tomorrow. The twins hear that the FBI has taken over the search for the bank robbers. Alicia doesn't return; everyone is frantic. Kit and Connie make a fruitless search for the musicians on the mainland. Connie tells Chip her suspicions of Sterling and Rusty.

Alicia is found and tells Connie that the robbers think that either she or Connie have taken the money satchel because it's missing. Connie locates the bank robbers and uses a ruse to lead them to the hidden cove where they're caught. The money satchel is still missing, but Connie remembers something Mr. Talmadge told her and discovers the missing brown satchel in an interesting place proving who really stole the satchel.