A robot flies into a scientist's secret lair and unloads a pile of cash into a vault. The robot is controlled completely from the scientist's command center, and many robots similar to it are lined up along the walls of the lair. The front page of the Daily Planet reports the "mechanical monsters" robbery right alongside an announcement for the display of 50 million dollars of the world's rarest gems at the local museum.
Later, as Lois Lane and Clark Kent are covering the museum's exhibit for the Planet, a robot lands in the street outside. The police pelt it with machine gun fire as it marches towards the museum, but the bullets bounce harmlessly off. Museum visitors, including Clark and Lois, flee as the monster marches towards the jewels and begins loading them into an opening in its back.
While Clark phones the Planet from the nearest phone booth, Lois climbs into the monster's back, just as the monster leaves the museum and takes off into the sky. Clark emerges from the booth, notices Lois gone, and says, "This is a job for Superman!" He goes back into the phone booth and changes his clothes, emerging in his classic red-and-blue costume.
Flying high above the city, Superman spots the robot and uses his X-ray vision to see Lois inside with the jewels. He lands on it and struggles to open the door in its back, only to have the scientist maneuver the robot upside down and throw him off into a power line, tangling him in the wires. As the robot is upside down, the door flies open and all the jewels fall out in the process, with Lois surviving only by hanging for dear life until the robot flips back over.
As Superman struggles to free himself from the wires, the robot arrives at the lair, but instead of jewels, the scientist finds Lois in its payload. Infuriated, he demands that she tell him where the jewels are. The next time we see her, she is bound and gagged on a platform held over a pot of boiling metal in part of what appears to be an industrial foundry. The scientist pulls a lever which starts some machinery gradually lowering her closer and closer to the liquid.
Meanwhile, Superman frees himself from the power lines and knocks down the door to the scientist's lair, only to meet the army of robots (numbers 1 through 27 are seen). Under the scientist's control, the robots emit fire from nozzles positioned on the lower part of their heads, encircle Superman, and pound him with their fists. Initially, the robots seem to have the upper hand, beating Superman to the ground, but Superman defeats them, sending the scientist running. When Superman catches up with him, he is holding a knife to the rope holding Lois's platform above the molten metal, and threatens to cut it if he takes another step. Superman makes a move, the rope is cut, and Superman speeds across the room to catch Lois just in time, landing on a ledge below the pot of molten metal and the scientist. The scientist then pulls a lever to dump the hot liquid on them, but Superman shields Lois with his cape, then grabs the scientist and flies from the lair to take him and Lois back to the city. The next issue of the Planet describes the latest adventure that the mechanical robots are finally destroyed, the jewels are recovered, and the scientist is finally sent to prison for the thefts. In the office, Clark says "That's a wonderful story, Lois." She replies, "Thanks Clark, but I owe it all to Superman." Clark smiles.Bud Collyer as Superman/Clark Kent, Police Officer, Inventor
Joan Alexander as Lois Lane
Jackson Beck as the Narrator