Mulder and Scully take out another vacation (much like in Ice) but again come across a deadly life form which seems to breeding in the lava of a volcano. Geez, you try and have a nice relaxing break and instead you almost get killed!

Scientists at a base pick up a distress signal from another group of scientists on the volcano and when they view it they clearly see one of their men lying dead and a strange shadow covering him.

Back in Washington, Scully is back in business after her abduction ordeal but Mulder is worried about her saying she should "give herself some time" but Scully wants to get back to work straight away. The pair set off for the volcano site along with another research scientist named Pierce but as soon as they arrive, Mulder gets attacked by one of the members of the research team. The survivors from the volcano, O' Neil, Tanaka and Ludwig suspect that their leader, Trepkos has gone crazy and is stuck on the volcano and they also say that he was the one responsible for the murder of the man seen on the video-tape.

Mulder investigates the site and comes across notes that Trepkos had made which mention a new kind of life form, one that is silicon-based and can survive the immense heat within the volcano. The notes intrigue Mulder. Meanwhile, Pierce decides to survey the site outside and try to look for Trepkos but Trepkos catches him and chokes him saying that "No one can leave."

Back at the research centre, Tanaka collapses unexpectedly so the team rush him off but as they do this he gets up and runs away only to fall down a steep hill. As Mulder and one of the other's watches from a distance, a spore (looks like a kind of snake thing) shoots out of his throat. The other one tells Mulder to keep away or he will be infected with the organism.

As Scully researches the spore, Mulder and Ludwig set off to find Trepkos and the volcano. They do locate him but Trepkos immediately fires a flare into Ludwig's body setting him up in flames and killing him instantly. Mulder is shocked but Trepkos simply says that it's not Ludwig he was trying to kill, but the thing inside him as Mulder watches it move around in Ludwig's neck. It seems that the robot, Firewalker which is able to search volcanos and bring back footage of what it finds, brought back the organism which infected the group except Trepkos who managed to escape. Mulder, realizing Scully is in danger as she was left with O' Neil, rushes back to save her.

Scully finds in her research that the spore must be inhaled straight away as otherwise it will become harmless. O' Neil enters and she cuffs herself to Scully but she manages to save herself by trapping O' Neil in a glass chamber seconds before the spore comes shooting out against the glass.

Trepkos returns with Mulder and is sad to see that O' Neil is dead because he was involved with her. When rescuers arrive, Mulder tells them there were only two survivors as Trepkos carries the love of his life down to the caverns below....