Mulder first became interested in aliens when his sister was abducted by one. Or so he thought. The more he worked on cases of the unknown, the more it became apparent that the disappearance of his sister was closely related to his parents' involvement in a conspiracy between humans and aliens for control of the earth.

Mulder's dedication to uncovering the truth about his sister became a source of increasing alarm to his parents, especially when he came to the attention of a shadowy individual known as the Cigarette Smoking Man. Not only is the CSM an aggressive member of the conspiracy, he was also an old friend of Mulder's father, and an even closer friend of Mulder's mother.

Mulder's work on the X Files have diverted him from his promising career path at the FBI. Once a high-flyer, he's now confined to the basement.