Daniel comes face to face with the Ori, a fiery race of beings who demand the worship of mortals. Elsewhere, Stargate Command encounters the first Ori missionary in our galaxy.

Samantha Carter returns to Stargate Command when the Ori seize control of a planet, using an expanding force field to gain a foothold in our galaxy.

SG-1 investigates when evidence suggests that there are still Goa'uld hiding on Earth -- including a former System Lord. Tensions between Earth and the Free Jaffa continue to mount.

Earth is infected with a deadly Ori plague, prompting an ally from SG-1's past to come to their aid. Gerak proposes that the Free Jaffa follow the Ori religion.

As the Ori plague spreads rapidly, SG-1 hopes that the Prior who caused the disease may hold the key to its cure. Gerak tries to rally the Jaffa to the Ori's cause, prompting Teal'c and Bra'tac to initiate a resistance.