“The Fourth Horseman: Part 1” is episode ten of season nine of Stargate SG-1.
A Prior of the Ori, who was the human who set fire to Vala back in “Origin”, has managed to convert Gerak, the leader of the Jaffa, to Origin, and he is going to spread the word of Origin to all the Jaffa as a state religion. Carter has come up with a possible way to defeat the special abilities of the Priors, using the information garnered from observing Anubis’s genetic experiment, Khalek, in the previous episode, “Prototype”.
Some SG-6 team members who were spying on a Prior have been infected by a plague which has escaped the base and spread to Colorado. Teal’c and Bra’tac are planning to stop Gerak – by whatever means necessary.
Orlin, the Ascended Ancient that took a liking to Sam in “Ascension”, is also hanging around, but he looks rather different. He’s a boy. He also reveals some of the history of the Ancients (back when they were Alterans, before they Ascended) and the Ori, as well as the truth about Origin, and how the Ori get power from their worshippers.
With a plague spreading and the Jaffa possibly converting to Origin, things are looking bad. Not that surprising in an episode named after one of the Four Horsemen of the Apocalypse.
The story continues in “The Fourth Horseman: Part 2”.