When a terrorist bomb destroys a building in Dallas, Texas FBI agents Fox Mulder and Dana Scully are drawn into a dangerous conspiracy surpassing anything they’ve ever encountered. With the dubious assistance of a paranoid doctor, Mulder and Scully risk their careers and their lives to hunt down a deadly virus which may be extraterrestrial in origin — and could destroy all life on Earth.
Mulder is less than thrilled when an Office of Professional Conduct board refuses to reassign him and Scully to The X-Files and even more so when he learns that Agent Spender and Agent Fowley have become their successors. When Gibson Praise reappears and comes to them for help, Mulder and Scully attempt to continue their quest undercover despite having fewer friends and less access and protection than ever before.
Mulder is trapped in a car with a man who has developed a serious ear condition which can only be suppressed by driving west at high speeds. As Scully tries to find the cause and a cure for the problem, Mulder and the man get closer and closer to the ocean.
Mulder goes to the Bermuda Triangle when he learns that the Queen Anne, a British luxury liner which disappeared during WWII, has reappeared in the middle of the Sargasso Sea. Mulder's boat is wrecked and after floating in the water, he is hauled aboard the ship which has just been hijacked by the Nazis searching for the man who will build the atom bomb. Mulder tries to convince the crew that they have traveled into the future, but evidence further suggests that it is he who is back in the past.
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.
Mulder is thrown in jail at the Area 51 compound but is released when it is discovered that the flight data recorder he stole was a fake. Scully comes to her senses and realizes that the Mulder she sees isn't who he really is and heads back to Nevada to help the real Mulder. Meanwhile, the mechanism that caused the body swap is rapidly snapping back, undoing everything in its wake, and Mulder and his alter ego must race to put themselves back where they belong.
Mulder talks Scully into investigating a haunted house on Christmas Eve where several couples have met their fate on that very night. While there, they encounter endless tricks and traps set by a ghostly couple who originally made a lovers suicide pact in the house. The ghosts try to convince Mulder and Scully to kill each other.
After a scan of a couple's unborn child is found to have strange growths protruding from the head and back, the fetus is taken from inside the mother by a demon creature while she is asleep. Mulder heads to investigate after Agent Spender dismisses the claims, and discovers that the father is actually a demon who wants to have a normal child.
Strange weather phenomena in a small town leads Mulder to believe that a weatherman's deep emotions and unspoken love toward his workmate are becoming real in the form of snow and tornadoes.
Mulder and Scully have 24 hours to save A.D. Skinner from being killed by a biologically engineered disease. The disease appears to be created by a shadowy government organization, but in order to save Skinner's life they must determine who wants him dead and why. With Skinner's condition worsening, Mulder seeks assistance from Senator Matheson, who he hopes can provide some answers and help him to find a cure.
Assistant Director Kersh tries to tear Scully and Mulder apart when he partners Scully with a New York agent investigating Alfred Fellig, a freelance police photographer who conveniently shows up to document the scene of a death as soon as it happens. While the young hotshot agent is convinced that Fellig is murdering the people, Scully has other (paranormal) theories.
While the Cigarette-Smoking Man recites to an unseen listener everything about the work of the Syndicate involving a global conspiracy in cooperation with an alien species, Cassandra Spender reappears and is pursued by the scientists who would rather kill her than keep her alive. Cassandra runs to Mulder's apartment and demands that he shoot her because she is the culmination of 50 years work - a living alien/human hybrid - and colonization will begin if the aliens find out about her.
The Cigarette-Smoking Man reveals all the government secrets from the past 50 years in order for his son, Jeffery Spender, to join him in his work. Mulder learns that the final stage of the conspiracy will soon begin with the hybridization of the Syndicate members, and he must stop it from happening. Agent Spender deceives his father and arranges for Mulder and Scully to be reassigned to the X-Files, which doesn't sit too well with the Cigarette-Smoking Man.
Arthur Dales summons Mulder and Scully to his holiday home in Florida where he is concerned over the disappearance of his neighbors. Despite the raging hurricane, Mulder and Scully head out to investigate and discover several strange deaths caused by an octopus-like sea creature that swam in through the overflowing sewer system.
A woman is forced to relive the same day over and over as she tries to prevent Mulder and Scully from being killed by her boyfriend during a bank robbery attempt. As the events restart again and again, Mulder's sense of Deja-vu grows stronger.
On their first official case back on the X-Files, Mulder and Scully go undercover as a married couple at a prestigious planned community where several residents have recently disappeared after failing to comply with the rules and regulations.
A mysterious dog thought to be the last of a rare Asian breed is responsible for several murders which Mulder and Scully are investigating. Now the agents have to find a way to stop the dog before it kills again.
After a tornado rips through a prison camp, one of the inmates disappears from solitary confinement and the prison warden is killed. As Mulder and Scully track him, he appears to have the power to pass through solid matter while changing its composition.
A writer living next to Mulder becomes obsessed with Scully and confesses to her that he is using her for a character in his novel. Scully finds herself strangely drawn to him even though he is now the prime suspect in the murder case that she and Mulder are investigating.
Mulder meets with Agent Arthur Dales' brother (also named Arthur) and is told a tale of a talented negro baseball player from Roswell, New Mexico in the 1940's who may have been an alien that ran away from his colony because of his love for the game.
While on a stakeout of a Government Defense Convention in Las Vegas, The Lone Gunmen are reunited with Susanne Modeski, the mysterious woman who disappeared ten years earlier when they first met Agent Mulder.
When the bodies of a couple who have been missing for less than a week are discovered to have been reduced to bone, Mulder suggests that recent paranormal phenomena in the area may have caused the accelerated decomposition.
Two scientists confer about each of their metal objects that were found in Africa and they theorize that the objects are pieces of a puzzle which could be undeniable proof that human, plant, and animal life did not originate on this planet. When one of the scientists is killed and the objects are stolen, the agents investigate and Mulder begins to experience severe neurological pain caused by the objects.