Dr. Dutro and the alien Suboko warn the Challengers that an invasion force has placed four machines across the planet to trigger natural disasters and soften Earth up for invasion. The Challengers try to destroy the devices, but can only move them. When they return to Challenger Mountain, they learn that Dr. Dutro was a fake and that Suboko works for the invasion force and that the devices protected the Earth. The Challengers restore the devices to their spots, despite an attack by aliens on camels with ray guns. They use the last device to drive the aliens back until the Space Police arrive.; Prof and Rocky are captured by fish-men, the Martinels, and try to protect them from their lava-men enemies, the Lavanites. Meanwhile, Ace and Red dive to find their buddies and encounter the lava-men. Both sides, fighting over old underwater oil pipes, plant explosives to fracture a volcano, unknowingly endangering each other. The charge goes off and the two teams bump into each other and see that they are working at cross-purposes. They insist that since both lava- and fish-races can wipe each other out, they better quit fighting. The two races make peace and the Challengers head for home.