Lieutenant Stepashin was an officer on board the Soviet submarine, the Firebird. He believed that the Cold War "[wouldn't] stay cold forever" and was a fiercely patriotic man. He was also stubborn, only willing to believe what he thought was the truth; he refused to believe the Eleventh Doctor and Clara were time travellers, and that Skaldak was an alien.
When the Doctor and Clara arrived on board in 1983, he ordered them to be searched. He attacked the Ice Warrior Skaldak with a cattle prod from behind, knocking him out. When Skaldak escaped capture and threatened him, he offered to form an alliance to "win" the Cold War. Stepashin told him about the Cold War and how both sides would destroy each other if one side attacked. Skaldak then killed him. (TV: Cold War)