The entire crew of the submarine 'Keppel' are found dead at their posts. All have died of cardiac arrest. The Champions join a fresh crew to re-trace the voyage but in the Atlantic they lose radio control.They find an uncharted island which is being used for illegal nuclear tests by a country whom the U.N. denied nuclear weapons. The 'Keppel' crew were killed because they came across it. Richard and Craig join a landing party but they are all gassed, except the two Champions, who are taken to be experimented on by mad scientist Minoes.The lads escape but must flee the island before it is engulfed in poison gas.