When investigating a case involving the murder of identical doctors, the agents come across a shapeshifting Alien Bounty Hunter (Brian Thompson) responsible for executing a series of alien clones. During this case a grown woman claiming to be Mulder's sister Samantha appears, telling Mulder of the Bounty Hunter's objectives and that she has the ability to identify him. When Scully is kidnapped by the Bounty Hunter, Mulder is forced to trade Samantha for her. During a botched attempt to kill the Bounty Hunter, Samantha is killed. However, it is discovered that this was simply one of many alien clones of Samantha. With the help of X, Mulder pursues the Bounty Hunter to a submarine in the Arctic. Mulder is nearly killed when exposed to the Bounty Hunter's toxic blood, but is saved by Scully.