Doggett and Monica look like they are working on good old fashioned FBI type surveillance when Monica manages to get cut and bleeds a lot, while Doggett goes after the killer, points a gun at him, only to have the suspect disappear at the blink of an eye, and reappear behind him… complete with Monica’s gun aimed at him. With that, Doggett gets shot and we cut to events happening simultaneously on the other side of town in Monica’s new apartment, with Agent Doggett there.
The rest of the episode focuses on parallel universes where Monica and Doggett each have a twin running around. In the middle of it all is the killer able to jump between them. Doggett believes he witnesses Monica getting killed, and Monica believes she was with Doggett when he was shot - despite the evidence accusing her. Monica has one way to prove it’s all a big load by unplugging Doggett’s life support machine, which she does, only to appear in her apartment with Doggett alive and well. Rather tearfully, they hug.
Scully pops in every now and then to say hello, and still no sign of Mulder.