The series revolves around Federal Bureau of Investigation (FBI) special agents Fox Mulder (David Duchovny), and Dana Scully (Gillian Anderson) who investigate X-Files: marginalized, unsolved cases involving paranormal phenomena. Mulder believes in the existence of aliens and the paranormal while Scully, a medical doctor and a skeptic, is assigned to scientifically analyze Mulder's discoveries, offer alternate rational theories to his work, and thus return him to mainstream cases. Early in the series, both agents become pawns in a larger conflict and come to trust only each other and a few select people. The agents also discover an agenda of the government to keep the existence of extraterrestrial life a secret. They develop a close relationship which begins as a platonic friendship, but becomes a romance by the end of the series. In addition to the series-spanning story arc, "monster of the week" episodes form roughly two-thirds of all episodes.