Mark and Fitzhugh are captured by a giant after witnessing a bulletproof man kill a policeman during a break-in. The giant, Professor Gorn, is a scientist who was monitoring his creation: a robot. They want Mark to lend his technical expertise to iron out the little bugs like the fact it kills people. Gorn and his assistant Zoral use Fitzhugh as a test experiment to prove Mark's theory that the robot has the equivalent of a defective adrenal gland.
Meanwhile, Steve, Dan, and Valerie break in using their laser drill. Mark eventually irons out the bugs. Steve and Dan help Mark escape, and all three take refuge in the robot. Fitzhugh deliberately gives away Valerie's presence, then spins a tale about how the Earthlings are master saboteurs. That gives Mark time to rig the robot so he can run it by remote control.
When Gorn calls in a Council member to witness his triumph, Mark has it free Fitzhugh and Valerie, then attack its creator and destroy the lab. Gorn is killed, although Zoral escapes.