In Contra III: The Alien Wars, players assume the role of either Bill Rizer or Lance Bean, who are members of an elite special forces unit known as the Contra Force. These characters are battle-hardened soldiers equipped with powerful weapons and armor suits. Players can choose to control either Bill or Lance to fight against the alien invasion and save the Earth from the nefarious forces threatening to destroy it. The choice of character does not significantly impact the gameplay or storyline, as both characters have the same abilities and arsenal at their disposal.