FBI agent Fox Mulder started investigating the X-Files that deals with unsolved cases of the paranormal. However, they couldn't be explained and were never proven by the FBI as they don't know the exact truth of what's going on. Mulder attempts to find out what happened to his sister from the day of her alien abduction at young age. This motivated Mulder to start solving the X-Files and later realize the government may have something to do with his missing sister.
Later, a new FBI agent named Dana Scully is hired to join Mulder to solve the unknown. However, Scully is a skeptic and wouldn't believe Mulder of what is actually taken place. But Scully adds a fine addition to solve the X-Files by performing autopsies on victims who were related or died by paranormal causes.
The X-Files is a group of unsolved cases that deals with the paranormal. Most of the cases deal with alien abductions and UFO sightings. Other cases deal with the supernatural where ghosts have attacked a family member or the demon possessing a child. There were other cases that deal with superhumans as they were rumored to display their powers and the existence of alien-human hybrids.
The X-Files was led by only two members of the team, Fox Mulder and Dana Scully in the first seven seasons of the television series. In Season 8, a new FBI agent named John Doggett joins the X-Files team after the alien abduction of Fox Mulder. This time, Scully plays the true believer of the unknown while John is the skeptic. Later, an FBI specialist named Monica Reyes joins the X-Files team.