When feared Yankees slugger Cliff Bullard goes barnstorming around the northeast, offering $10,000 to any local pitcher who can strike him out, Professor Childermass and Johnny Dixon get a sneaky idea. There's a local legend about a crackpot inventor who once built a robot capable of throwing a baseball 110 MPH, and the professor thinks that if they find the machine, they can win Bullard's prize. They discover the rusted old monstrosity in an abandoned workshop and put it back together, piece by piece. But when they screw in the robot's eyes and it comes to life, they realize they have made a terrible mistake.

As soon as it's activated, the robot attacks, trying to kill Johnny and the professor. Was it made to be a killing machine, or have its circuits been corroded? To save the town and get a crack at the $10,000, Johnny and the professor will have to tame the steel beast.