The US Navy informs Captain America that Hydra has captured the U.S.S. McKenszie, loading a powerful virus into its navigational computers. It is currently in a communication dead zone, but once it is able to reconnect, the virus will be uploaded to every ship in the naval fleet, preventing them from stopping Hydra's announced attack on the United States. The Admiral is left with no choice but to sink the submarine and the entire crew to save millions of lives.
Cap offers his assistance and sneaks aboard the submarine. Working with the crew, they fight back against the terrorists. With no time remaining to stop the bombardiers, Cap orders the sub to dive and destroys the controls keeping them off the network.
With the commanding officer wounded, Cap finds Lt. Commander Rebecca Houston and works with her to destroy the sub. It's the only way to prevent this sub from taking out the naval network. After all personnel have evacuated, they arm the torpedoes and begin the countdown. Cap and Houston climb inside a torpedo tube, planning to ride the explosion back to the surface.
When the torpedoes detonate, they are blown free from the sub, but the force is insufficient to allow them to reach the surface. Cap quickly realizes that he has lost his grip on both his shield and commander Houston. Cap locates Houston first and tries to reacquire his shield as it continues to fall deeper into the ocean. He soon finds that he is unable to pursue his weapon without jeopardizing Houston.
Cap abandons his shield and surfaces with Commander Houston. They find the remaining crew aboard life rafts, climb aboard, and wait for assistance. Cap silently morns the loss of his shield.