Stryker's Island, a prison situated just off the Metropolis Coast. A guard wheels a cart filled with food into the cell of a man named John Corben, who is imprisoned for stealing LexCorp's experimental war suit and battling with Superman, causing massive damage to the city. His room is filled with expensive paintings and opulent furnishings. When the guard asks how Corben managed all this, Corben smugly responds that he knows the value of good friends, and how to keep his mouth shut. As he passes the guard a sum of money as a "tip" Corben begins to sweat profusely, and doubles over in pain, eventually passing out on the floor. In the hospital wing of the prison, Doctor Vale tells Corben that he has a rare disease, and will die soon. Corben is distraught, but Vale tells him not to worry, that they have a mutual "friend" who is watching out for him.
Sometime later, Clark Kent is bicycling past the ocean when he sees two missiles streak across the sky and blow up the outer wall of Stryker's. Quickly changing into Superman, he deflects any other missiles that go for the prison, and borrows a fisherman's net to round up all the inmates who are trying to escape via the water. He even uses a massive boulder to block the hole in the wall. But Corben has already escaped using scuba-diving equipment.
In a laboratory, Lex Luthor is praising Corben for his loyalty, since he didn't rat out Lex's part in the LexSuit scam. Corben rebuffs, saying Lex's plan is some way to repay him, sarcastically. Lex claims that Corben would have died anyway, but if he allows Lex to "help" him, he will not only live, but he will have power enough to kill Superman. Lex has proposed to put Corben brain into an android body composed of a rare metal called "Metallo" and powered by a large chunk of Kryptonite. Lex even promises Corben that he will look exactly the same. Corben agrees, since he feels he has nothing to lose.
As news of Corben's escape becomes public, Lois tells Clark that Corben won't enjoy freedom for long, and shows him Corben's medical record, which tells them that he was dying from a virus. Clark is shocked at how rare the disease is, but Lois points out that Corben was a mercenary who had been all over the world, he could have picked it up anywhere in his travels.
Next we see Corben, standing in front of a mirror, looking exactly the same and telling Lex the he has never felt better. They test Corben's strength on a punching machine, but his attack is off the scale. Corben's next punch decimates the machine. They fire a bomb at him, and he takes no damage. Lex tells Corben that even his "skin" is laced with "metallo." But Corben remarks that he can't feel anything, like he is controlling his body from a distance, and that he hasn't had bodily needs like hunger since he awoke from the operation. Lex claims that there are still a few adjustments that need to be made, but tells Corben that his current objective should be taking out Superman.
At the Train Station, Corben realizes he can't even smell the rose a woman tries to sell him. He jumps on the tracks and collides with the train, causing massive damage in hopes of attracting Superman, who shows up soon. The two battle, tearing through Metropolis, evenly matched, it would seem. That is, until Corben opens his chest place, revealing his Kryptonite "Heart." Corben proceeds to wail on a weakened Superman until Lois drives her car between them, trying to help. Metallo advances on Lois and kisses her. He realizes he still can't feel anything. While he is distracted by this revelation, Superman climbs into Lois's car and hit the gas, plowing into Metallo and knocking him off the bridge, and giving Superman a chance to heal.
Corben storms into Luthor's Lab and demands those upgrades Lex promised. The confused scientist tells Corben that the adjustments are his own to make, meaning he is going to have to get "adjusted" to being a machine. The scientist tells him the process is non-reversible, but tries to play up the good, claiming that Corben now has virtually limitless strength and can't feel any pain. "What good is that if I can't feel any pleasure either?!" Corben bellows, before stalking into the bathroom. He looks in the mirror and proceeds to rip off portions of his own "skin," revealing the shiny metal skeleton underneath. He claims that this is all he really is, "Metallo!"
Meanwhile, Clark and Lois are on Stryker's Island, going through the offices of Doctor Vale, who has recently retired and disappeared. Clark notices a strange looking bottle of liquid, and Lois finds a parking pass for LexCorp.
In the harbor, Lex is confronted by Metallo on his yacht. Metallo rages about being unable to feel anything and threatens to kill Luthor. Lex tells him that there may be no solution now, but that he has his people all over the world looking into this, and that it may take time, but what does he care, he's practically immortal. Lex tells him he's going to take him somewhere safe and takes them out to sea. Superman finds them on the yacht and confronts Luthor about his connection to Doctor Vale. Lex implies that the doctor is already dead. Metallo attacks Superman from behind. Metallo tries to kill Superman, who in turn tries to stay out of Metallo's reach long enough to reasoning with him. Metallo fires a concentrated beam of Kryptonite at Superman, who crashes into the boat. As Metallo is about to deliver the finishing blow, Superman tells Corben that he was in fact poisoned by Doctor Vale on Luthor's orders, and that's how he got the virus, Superman shows him the bottle he found in Vale's office. Metallo turns on Lex and Superman blows up the yacht's gas tank with his heat vision, blowing Luthor and Metallo into the water. Lex finds a life preserver, while the completely metal Metallo sinks to the bottom of the ocean. Superman saves Lex from a shark, and carries him back to shore. Lex tells Superman that he can't prove Lex had anything to do with Corben's illness or Metallo's creation. And that even if he could find Corben to use as evidence, there would still be the kryptonite to worry about. Superman says "I don't think I'm the one who should be worried." As he flies away, Luthor looks out onto the vast ocean.
On the ocean floor, we see a slight green light far away that turns into the shape of a man, walking the great distance deliberately and purposefully toward land...