The Red Skull has trapped Captain America inside the Cosmic Cube. He is currently living in a fantasy world created from his memories. He relives the day he received the super-solider serum, only this time he is able to save Professor Erskine from the Nazi saboteur - with his trusty sidekick Bucky Barnes at his side.
The Skull watches these activities from outside the Cube. He is content that his plan to trick Rogers worked as well as it did. Sharon is disgusted at her part in the Skull's plan. She agreed to work with the Skull to eliminate Hitler's consciousness from the Cube and now regrets their alliance. While the Skull marvels at his master strategy, Sharon suggests that he recruited Rogers to do a job he lacks the courage to do himself. The Skull punches her for her insolence. His victory is short-lived as the soldiers from the KubeKult find them. They retreat to a more secure room to wait for Cap to defeat Hitler.
Inside the Cube, Cap and Bucky fight their way through the Nazi soldiers trying to find Hitler. Before Cap realizes it, they are back in New York circa 1945. He recognizes it as the neighborhood he grew up in. The televisions in a nearby store window report of the victory in European Theater. The broadcast is interrupted by saboteurs who have commandeered the Empire State Building. They plan to use this as their new base to help take over America.
Cap and Bucky quickly arrive at the Empire State Building and chase the Nazi invaders up to the roof where they find a waiting airship. Despite Cap's insistence, Bucky climbs the ladder into the ship. When Cap follows, he finds it occupied by Baron Zemo. He has captured Bucky and tied him to "the" drone plane. Zemo lauches the plane, but in this reality, Cap catches it and saves Bucky. They return to the ship, but it has been abandoned.
They plot a course for Germany to bring Hitler to justice. Cap and Bucky rendezvous with Nick Fury and his Howling Commandos in Berlin. They are joined by Namor and The Human Torch. Working together they clear a path to Hitler's bunker. When Cap stands on the precipice of achieving his mission, Bucky refuses to continue.
Bucky informs Cap that he is just part of his subconsious mind at work. All along anachronistic items have been popping up to remind him that he is not in the right time. He has been manipulated by the Skull. If he succeeds in his mission and kills Hitler, the Skull wins. He has to fight his instincts and will his return to the real world.
Moments later Cap materializes outside the room occupied by the Skull and Sharon. Cap breaks down the door and attacks the very surprised Skull, separating him from the Cube. As the Skull tries to reacquire the Cube, Cap warns him to stay away. The Skull ignores him. Cap throws his shield at the Cube, severing the Skull's right arm before destroying the Cube. The resulting explosion disintegrates the Skull, leaving only a dark silhouette on the nearby wall.
Cap and Sharon round up the remaining Kultists and leave the compound. Cap offers her a ride back to SHIELD to uncover the reasons she was abandoned all those years ago. She accepts, but states that she has no interest in resuming their relationship. She wants the same answers he does. Cap halfheartedly accepts her answer and they drive away in a jeep.