Mulder and Scully investigate mysterious events at a scientific project using a robotic probe, "Firewalker", to explore the interior of a volcano.
- Chest Burster: The alien fungus-like organisms grow in human chests, though it ultimately bursts through a victim's throat.
- Continuity Nod: Mulder is hesitant to bring Scully along due to the events surrounding her abduction, but she insists she's ready.
- My God, What Have I Done?: Mourning her death, Trepkos recalls how he convinced Jessie to join the operation to "have an adventure."
- Not Himself: The guest characters insist this about Trepkos.
- Silicon-Based Life: The alien organism that Mulder and Scully went to check is not carbon-based, but silicon-based, meaning it's Not of This Earth.
- Teacher/Student Romance: Hinted at between Jessie and Trepkos, two scientists working at the station.