Players take on the role of Marvel superhero Captain America and must guide him around the Doom Tube base of Dr Megalomann. The base is a long tube inside another tube, splitting areas up into chambers around a central core. The virus has been released into the base, and players can only access areas below their immunity level. By defeating enemies they can collect ying-yang which boosts their immunity. Captain America can throw his shield to take out enemies, but the number of shields he holds are limited, although he can re-use those which kill enemies or if he recalls a shield before it is lost. As the player moves Captain America around areas they need to recover parts of a password which will allow them access into the final area to disable the missiles. The game is played in real time, giving the player only one hour in which to complete the game.
Source: Wikipedia, "Captain America in: The Doom Tube of Dr. Megalomann", available under the CC-BY-SA License.