Season 6, Episode 9.

Scully discovers Skinner's blood sample and, after checking, she finds that Skinner's blood contains some sort of multiplying carbon nanotechnology. Later, Mulder also arrives at the power plant and confronts Matheson. At the hospital, Skinner goes into cardiac arrest but suddenly revives when the bearded man deactivates his remote control. In the final scene, the bearded man appears in Skinner's car and is revealed to have been a disguised Alex Krycek, a rogue FBI agent who formerly worked for the Syndicate, and who continues to control the potentially debilitating nanotechnology in Skinner's body.