Steve Rogers brings a pizza over to the apartment of his girlfriend Bernie Rosenthal and is surprised to find she is a big fan of All-Star Wrestling on TV. They watch as wrestling trainer King Arthur, a champ himself until an accident robbed him of the use of his legs, presents his top wrestler, Jumpin’ Jack Flash, as the one to beat the notorious dirty fighter Mr. X. Steve and Bernie are shocked when the fight ends and Jack Flash is pronounced dead, killed by Mr. X’s signature Back-Biter Kick. A devastated Bernie asks Steve to go and he determines to see the killer brought to justice. As Captain America he visits the police, explaining that wrestlers are role models to kids and that good must triumph over evil. King Arthur arrives and announces he has received a call from Mr. X who claims to be innocent and offers to prove it in the ring—if he wins he goes free. Cap volunteers to be X’s opponent in the "trial by honor."
A week later Cap faces Mr. X in the ring at Madison Square Garden. As the fight begins, it seems that X is getting the better of his opponent but Cap reveals he was holding back to get the measure of his foe before easily overpowering him. The defeated Mr. X is unmasked as Ray Deacon who spent five years in prison, unjustly convicted of being the serial killer Midnight Slasher. Deacon is taken away to jail.
Over the next few days, Steve is unable to let the matter go; something seems wrong so he plans to investigate further. He stops by King Arthur’s offices and requests to look at pictures in the research library for an art job he’s doing on physical fitness. Arthur says they do not have a library and suggests Steve sketch some wrestlers in training. Leaving the office, Steve runs into Marty the ringside announcer who takes him to the tape library King Arthur claimed did not exist. Steve studies films of the two matches and concludes that, because of different fighting styles the Mr. X who killed Jumpin’ Jack Flash was not the same man he fought. He takes the tapes to show King Arthur and walks into a meeting of Arthur and a gang of hoods. Steve is seized by the thugs and Arthur reveals that he has been feeding his wrestlers illegal steroids secretly. Jumpin’ Jack Flash discovered this and threatened to go to the police so Arthur drugged Mr. X, donned his costume and killed Flash in the ring—the steroids having given him back the use of his injured legs. Arthur then binds Steve with a metal rod and dumps him in the incinerator chute. On his way down, Steve frees himself, stops his deadly slide and switches to his Cap outfit. Bouncing back to take on Arthur’s hoods, Cap and the steroid-enhanced King Arthur fight it out, with the wrestling master preaching the doctrine that might makes right. Cap crowns the King and announcer Marty, who heard the whole thing, offers to testify against him. Cap has Ray Deacon freed from prison and encourages him to go on with his life. Later, Bernie answers a knock at her door and finds a single rose Steve has sent.