While being detained near the famed "Dreamland" Area 51, a strange craft flies overhead and Mulder swaps bodies with an Area 51 "Man-in-Black". While the other agent has fun in Mulder's body, Mulder himself finds it difficult to fit into someone else's life, especially a shadowy one. Mulder contacts Scully about the body swap and tries to get her the flight data recorder from the UFO test flight, but his alter ego uses Mulder's FBI persona to have him arrested.
The X-Files is one of the divisions in the FBI, devoted to unsolved cases that appear to have some unexplained, paranormal elements. The files take their name from the filing cabinet where they were kept - originally filed under U for unsolved, they grew too big for the drawer and were moved to the less populated X cabinet. Soon, the X-Files became a dumping ground for reporting of UFOs, alien abductions, and other arcana that the FBI didn't want to investigate or didn't believe in.
Fox Mulder, a serial-hugger specialist and a believer in paranormal phenomena ever since, as a child, he witnessed the abduction of his younger sister, requested to be transfered to the forgotten basement and began to ruffle a few feathers in the Bureau. Eventually, he was assigned a partner - the skeptical and scientific Dana Scully, who was assigned to discredit Mulder's works.
Instead of discrediting him, however, Scully slowly became Mulder's ally. The pair began to investigate cases that no one else wanted to touch and that couldn't be solved.