The SG-1 team travels to P3X-797 and discovers it is divided into a dark and light side. On the light side live the Untouched, a Bronze Age civilization based on the Minoan Earth culture. The Touched, who live on the dark side, are heavy-browed savage primitives.
Once they've found out enough to make their report, SG-1 returns through the Stargate but Jack and Samantha, and then the rest of the base staff, start mutating into Touched as well. Only Teal'c and Jackson are immune - Teal'c due to his symbiote and Daniel for unknown reasons. With the base shut down in quarantine, they have no choice but to go back to the planet and attempt to find a cure. They eventually discover that Daniel's anti-allergy pills act as a blocker to the plague and return to work through the base and cure the staff, then give the cure to the Untouched to administer to their brethren.