Challengers of the Unknown [Ace Morgan; Rocky Davis; Professor Haley; Red Ryan]; Carl Jackson; Warren Moss; Commander Lockwood; Colonel Price. Villains: General Tolek; Garnat; and other Tyrans.
At the Pentagon, Navy officers and scientists are stumped. Two scientists mysteriously disappeared in Antarctica. They call in "men capable of handling the unknown." Ace, Rocky, and Red fly off, picking up Prof along the way. Off New Guinea, Prof is astounded to find a volcano rising from the sea bed. In the Antarctic, the Challengers snow-tractor toward the mystery spot. The tractor bogs in a snowstorm and needs repairs. Ace and Rocky explore and fall through thin ice! They land on a futuristic bridge. Above, Prof and Red are captured by skinny orange aliens in bubble-suits.
Prisoners, two Challengers are dropped down a miles-deep airlift. Ace and Rocky find an airshaft and search for miles. The gravity is lighter and they half-float. Finally they reach a vast underground super-city of aliens - with an army ready for invasion! Ace tries to capture a sentry, but the alien anticipates his every move with its computer-like mind. All four Challs are imprisoned with the missing scientists. The aliens are planting charges all over the world to blow away chunks and reduce Earth's mass, so they can live comfortably among enslaved humans. They've conquered whole galaxies this way. Prof realizes the undersea volcano was a test. The Challengers think frantically. How to outwit the aliens? Their super-brains must need rest. Catch one half-asleep and...
The Challengers summon an alien during the night and succeed in hypnotizing them. The docile alien leads them out a secret panel to a gravity control room. The Challengers crank the gravity high, pinning the aliens to the ground. How to escape? The aliens use "electronic transmission". Ace and Rocky will try to destroy the main detonator from this end. Prof and Red teleport to Washington to alert the military. Prof is given a wearable minisub and torpedo-dives to dismantle one planet-cracking bomb. The Air Force and Navy arrive off Antarctica and shell the hell out of the Tyrans' invasion force. Underground, Ace and Rocky and the two scientists flee a gas attack and gun-cars. One scientist remembers the power comes from a core tap. Cranking a valve brings raging heat that crisps aliens and slags the bomb detonator. Meanwhile, in the Pacific, a wild claw-machine erupts from the ocean to attack the Navy.
On the Pacific island, Red sprays a new secret chemical weapon, a corrosive gas that eats the machine. Soon Ace leads a fleet of bombers to pound what's left of the alien base, and the menace is over. Their only casualty is Prof, who caught a cold diving off the South Pole!