Inside his personal laboratory, John Henry Irons forges a new suit of armor descended from the design he created in the episode "Prototype." He is assisted by his gifted niece Natasha, who points out that he has spent so much time testing his new suit, she and Grandma never see him anymore. He says he wants the suit to be perfect, so he can start helping Superman fight crime.
On the TV, reporter Angela Chen is interviewing Bibbo Bibowski, who witnessed a diamond robbery that is estimated at several million dollars. Bibbo claims that one of the masked thieves broke down a brick wall with his bare hands, making him, in Bibbo's estimation, "Almost as strong as my pal, Supes!" It is believed the thieves had ties with Intergang. After seeing the broadcast, Irons is more determined than ever to perfect his suit, and invites Natasha to assist him with target practice.
Meanwhile a bank is being robbed by three masked men, one of whom, despite being of average size, is able to pull the vault door off it's hinges. As the other two begin to clean out the vault, one of the men gets mouthy with the super-strong one, saying that he should help with the job as he "owes" them. The strong-man lifts the other clear off the ground, saying that after this job, they are even. While they are distracted, the bank manager activates the alarm, causing the men to flee into the alley where the getaway truck is parked. As the cops pull up, the strong-man tells the others to leave him and go. He proceeds to block the cop cars, tearing them all to shreds.
As Irons and Natasha test the suit's gauntlet-based weapons, the criminals drive by with the police behind them. They manage to give the cops the slip, but Irons fires at them with the gauntlet, taking out one of their truck's tires. The thieves are less than pleased, and open fire on Irons, whose gauntlet is damaged in the fire-fight. As the criminals advance on him, Superman arrives to save his life. He praises Irons for his bravery and work on the suit, reiterating that if Irons can get it to work properly, he'd appreciate his help. Just then, Natasha notices the last of the three criminals coming toward them with his hands raised in surrender. As Superman moves to apprehend him, the Man of Tomorrow is surprised by the other man's punch, which sends him reeling. The thief removes his mask and shirt to reveal his true identity-as Metallo! The android tells Superman that he was found by Intergang, trapped in volcanic ash, and powerless thanks to Superman's removal of his Kryptonite heart. They fixed him up (but for some reason, replaced only half of his "human" face). He and Superman battle, and Metallo shows that he has been upgraded when he unleashes a concentrated blast of Kryptonite radiation from his eyes, bringing Superman to his knees.
Irons tells his niece to run for her life, grabs a metal rod and tries to come to Superman's aid. He fails, and Metallo tells him to get ready to be famous, as he is about to be forever known as "The Man who watched Superman die!" before opening his chest cavity and exposing Superman to a fantastic amount of pure Kryptonite radiation, nearly killing him. Just then, Natasha rounds the corner in her car and slams into Metallo, sending him flying several feet. When he finally get up, he sees that Irons has loaded Superman into the car and driven away. Metallo collects his cohorts and steals a cop car to give chase. They follow Irons and company for a time, but are finally separated by a garbage truck and Natasha's wild driving.
When they finally arrive back at Iron's lab, a large group of people gather to look at the damaged car, and are stunned when Irons emerges carrying an unconscious Superman. They all promise to help Irons hide the wounded hero. Irons and Natasha carry Superman into the lab and lay him on a table underneath a sunlamp Irons designed, which will flood Superman's body with the same yellow solar energy that gives him his powers, hoping that will accelerate his recovery. When Metallo arrives outside and receives no help from the local people in finding Superman, he tells his men to open fire. With Superman unconscious and his neighborhood under attack, Irons has no choice but to don his new armor and fight Metallo himself as the hero Steel!
Steel makes short work of Metallo's allies, and begins to battle with the android himself. Using a combination of his jet boots and muscle amplifying armor, and also given the fact that Metallo's best weapon, the Kryptonite, is useless against him, Steel seems to be an even match. The fight eventually spills over into Iron's lab. Metallo manages to get Steel's giant metal hammer away from him and prepares to deliver a killing blow, but just then, a recovered Superman grabs the android from behind, and pins him. Metallo has another trick up his sleeve, as his head turns completely around and fires his Kryptonite eye beams. Superman is down again, and Metallo is gloating when Steel picks up his hammer and hurls it with all his might at Metallo's chest, blocking the harmful rays. The momentum from the swing carries Metallo through a wall and out onto the street several floors down. He does not get up again.
As Steel looks to make sure the job is done, the whole neighborhood cheers and sings his praises. Superman climbs to his feet and says "Well, I'm sure you will hear this a lot from now on. But I think I should be the first to say it. Thank you." They shake hands and a new legend is born!