January 7, 1943: as an evening newscast describes the events of the day, Captain America and Bucky are speeding over the nighttime rooftops heading for a downtown hotel. Arriving at their destination, they sneak in a window and spy on a band of Nazi agents in the next room. Caught, they take out their foes and move to the bedroom where an old man lies helpless and more Nazis are setting the room on fire. Cap attacks the disguised leader and discovers he is Master Man, his enemy counterpart. The two spar a few minutes before the Bundist villain leaves through a window. The old man on the bed weakly mentions that his invention was stolen by the spies and points to a letter on a table—then dies from his injuries. Cap tells Bucky the man was the great scientist Nikola Tesla, whose work with electricity and radio changed the world. He had been working on a revolutionary defense system called the Teleforce and Cap hopes that wasn’t what the Nazis stole….
Later Cap and Bucky, with FBI Agent Winchell, view a newsreel profile of Baron Zemo, Nazi scientist who once collaborated with Tesla but returned to Germany with the advent of the War. They watch how he destroyed an entire village to escape capture by American soldiers (actually the Howling Commandos). Forced to wear a mask to avoid reprisal, he encountered Captain America and was splattered with Adhesive X, permanently gluing his mask to his face. Winchell explains that the Teleforce would emit rays to stop all aircraft in the sky, making a country impregnable. The only clue is in the letter found in Tesla’s room; it was sent from Madrid by one Janice Dare warning Tesla that his life was in danger. Steve and Bucky fly to Madrid (a neutral country in the War), where they track Janice Dare to a bullfight. Steve approaches her in the stands but she, thinking Steve is a German agent, denies everything and in her attempt to escape falls into the bull ring. As the bull turns to charge her, Steve quickly changes into his Captain America costume and leaps into the ring to wrestle the bull away from her. Knocking the bull out with a circular wooden sign, Cap refuses to slay the creature and instead hurries Janice Dare out of the ring. Outside he tells her he knows she is Baroness Hilda Zemo, who was believed dead. Though she will always love her husband and son, she passionately hates the Nazis, so she faked her death in a car accident and fled to Spain. She offers to lead Cap to Zemo if he promises to a) not kill Zemo, and b) try to bring her son out of Germany. Cap agrees, though Bucky suspects a trap….
The trio arrive in Berlin where Hilda reveals that Zemo is working in a secret lab near the zoo. [Meanwhile, Allied bombers set off on a raid on the German capital.] While Bucky stands guard upstairs, Hilda leads Cap to Zemo’s secret lair. Entering, Cap spies the villain sitting, apparently deep in concentration. Approaching him from behind, Cap discovers it is a dummy; the real Zemo, with Master Man and several soldiers have them surrounded. Cap tries to protect Hilda by claiming he forced her to lead him there, but she instead confesses her love for her husband while proclaiming her defiance of the Nazi regime. Zemo orders Master Man to kill them both; "Captain America first!" he shouts, and attacks. Because of the instability of the German Super-Soldier Serum, the Nazi strong man is weaker than he was and finds himself beaten down by the American hero. A guard shoots Cap through the shoulder and Zemo orders him and Hilda to be imprisoned while his new death ray, the Medusa Effect (so called because, like the mythical monster, it can halt planes in the air as though they were turned to stone), will destroy the Allied bombers….
Meanwhile, Bucky, on guard upstairs, has been spotted by a squad of soldiers and fired upon. He escapes from the zoo and heads to the Zemo mansion and enters an upstairs window to find young Helmut Zemo asleep. Bucky tells the boy his mother is still alive and in the Baron’s secret lab. The two youths hurry over there and sneak into the lab. There, Zemo outlines his plans to destroy the Allied planes and Captain America. Helmut betrays Bucky to the Nazis and the young hero is captured. Baron Zemo tells his son that Bucky was lying and his mother is dead and promises him he can be the first to fire the Medusa cannon. "Wunderbar!" says the boy….
Cap awakens in the dungeon to find Hilda applying first aid to his wounded shoulder. The two share an awkward kiss before getting down to the business of escape. With much persistence, Cap manages to batter down the steel door and subdue the astonished guards, though Hilda is wounded by gunfire. She appeals to him to stop Zemo and rescue her son if possible. Cap heads off alone to the lab where Zemo has the cannon in place and Bucky tied across the barrel to be the first casualty of the super weapon. Cap discovers his shield in the room and hurls it, knocking Master Man down for the count. Zemo, heedless of his sons safety, opens fire with a pistol. In the melee, the angle of the cannon is changed and Bucky awakens. Hearing the bombers overhead, Helmut fires the cannon—Bucky manages to free himself in time but the cannon’s changed angle blows the castle wall apart as the bombing raid begins. As the building collapses around them, Bucky scoops up the unconscious boy and they rush to Hilda’s side, only to find her dying, wounded by falling debris. Her last words are to inquire about her husband’s safety. The heroes make it to safety though young Helmut escapes in the confusion. With a mixed victory in hand, the heroes head home.