This episode is the conclusion to the mid-season break cliffhanger "The Fourth Horseman (1)." There are 3 (you can make the case for 4) storylines revolving in the episode. The first of course is Carter and Orlin's(a descended Ancient)race to complete a cure for the Prior plague. The major struggle is Orlin is slowly loosing his knowledge he had as an Ancient, at the cost of doing major damage to his human brain. The second storyline revolves around Teal'c and Gerak. In the cliffhanger Gerak was turned into a Prior and had pretty much succeeded in making the "Path of Origin" the religion of the Jaffa. Teal'c is out to stop him, not wanting to see the Jaffa bow down to more false gods. The last follows Mitchell and Jackson's testing of a new device to inhibit the Prior's powers and trying to turn him against the Ori. They succeed in capturing him and leaving him powerless. After long and hilarious interogating, the Prior is killed as his powers start to return, but not after revealing that the Ori are coming to this Galaxy. As this is going on Teal'c and Gerak meet to come to a peaceful solution to the division of religion in the Jaffa government. Teal'c leads Gerak to Gerak's father's tombstone. After an intense discussion Gerak sees the light and returns with Teal'c to Earth to cure the plague. Before he heals it he hesitates and announces that he will die upon doing it, a failsafe the Ori must have put in the Priors to keep them from turning. Gerak this sacrifices himself to cure the plague. Dr. Lam then creates a vaccine from the antibodies in the cured patient's blood which is used to cure the plague for the rest of Earth, and can be used to cure any other Prior plague. The episode ends with Sam visiting Orlin at some sort of center. It appears the Orlin has lost all his memories, including those of Sam.
This episode was one of the greats. The tides have turned against the Priors but the Ori are coming. It was truely a great conclusion to a great clifhanger. Full of Action, Hilarity, and Insight.