Mulder and Scully join forces with several experts to investigate an Arctic base following the deaths of a research team working there. It emerges that the team had drilled deep into the ice and uncovered worm-like organisms that have the power to infect people and make them violent and murderous towards others.
Scully wants to kill the organisms they have captured to prevent outbreak, but Mulder wants to analyse the specimens to learn more. This leads to a loud argument and others suspect one of the pair is infected - although both pass their blood tests.
At night, Mulder follows a trail of blood, opens a freezer and a slain corpse falls on him. In walks Scully and other crew members, one of whom accuses Mulder of murder and being infected. Despite the FBI Agent's denials, Scully insists on him taking another blood test and demands that he put his gun down. Mulder feels that the results will be doctored and refuses to comply.
Mulder points his gun at a crew member when he grabs a crowbar, prompting Scully to pull her gun on him.
"Scully - get that gun off me!"
"Mulder - you have to understand!"
Suddenly, Mulder pivots round and directs the gun at his colleague.
"Put it down!" he screams.
"You put it down first!"
"For God's sake it's me," he responds, adopting a calmer tone.
"Mulder, you may not be who you are"
After a few seconds of stand off, Mulder lowers his gun. She later locks him in a room, but not before he tells her: "In here I'll be safer than you."
Scully devises a plan to wipe out the two worms by making sure they are both in the same host body and thus will wipe each other out. The pair examine each other and convince each other of their own wellbeing. Soon, a female crew member is unmasked as the host and she is restrained while another worm is placed in her body. The plan works. However, Mulder is certain that the real threat is still lurking deep beneath the ice.