Mulder and Scully learn that they aren't the only ones desperately searching for their long-lost son, William. The very fate of the world may depend on the outcome.
Mulder and Scully investigate a case involving several deaths where the victims previously reported encountering their own doppelgangers. The clues lead to a pair of fraternal twins playing a dangerous game telepathically.
Exploring the idea of The Mandela Effect, in which large groups of people remember an alternate history, Mulder and Scully find out how the X-Files themselves may really have originated.
When a pair of teenage girls attack one another, each believing the other to be a monster, Mulder and Scully find that their investigation could possibly lead back to their long-lost son, William.