After a powerful storm, Ty-Gor, Malma, and Junior De Snook wash up on the shore of a Tropic Island. There, they discover a Nazi Submarine Base. Looking into the unmanned base, the castaways discover a retrieve a Nazi torpedo and set it up on the overlooking hill in a swing built with vines.
A U-2 Nazi submarine appears and the crew comes ashore. Their commander orders the radio operator to listen for communications from ships to target. While the crew is relaxing, the jungle boy and Junior sneak aboard the submarine. Ty-Gor turns the u-boat's Submerge Wheel, causing water the ship to submerge as the lads leap out of the open hatch while the ship sinks.
The radio operator receives a signal from Captain Nickel of the freighter named the 99¢ Steamship Line. The commander is informed, but he finds his submarine is gone. Junior laughs at their situation, alerting the Nazi crew to their presence. As the Nazis head up the hill, they see the boys with the torpedo swing. The commander orders the crew back to the munitions shack below to retrieve guns. When the crew gets there, they see Malma sitting on top of the shack. The sailor's surrender. When Captain Nickel sees the situation through binoculars, he disembarks and takes the Nazi crew captive, placing the prisoners in cages meant for transporting animals. A group of blood-thirsty cannibals observes the situation.