SG-6 returns from P2X-885 with bad news: The planet, like so many others, has surrendered to the Ori and to their ambassadors, the Priors. Soon afterward, Teal'c learns that Gerak, leader of the Jaffa council, has also pledged his allegiance to these new false gods. Faced with these increasing threats, Carter becomes adamantly determined to design a weapon that can defeat the Priors' superhuman powers. Suddenly, a member of SG-6 collapses with a severe illness. Dr. Lam recognizes the symptoms: They match the plague that the Priors unleashed on P8X-412. What's worse, another member of SG-6 has already left the base, carrying the highly contagious disease outside. Mitchell sets out with a hazmat team to control the damage, but from the beginning it's a losing battle. Meanwhile, Teal'c and his old mentor, Bra'tac, head for their homeworld, Chulak. Though they don't want to start a civil war with their Jaffa brothers, they reluctantly gather supporters to resist the Jaffa nation's conversion to Origin. Even so, they still fail to appreciate how far Gerak will go to ensure the success of his new crusade. Back on Earth, the infection spreads quickly to neighboring states. Things look bleak. Then an unexpected old friend visits Carter - Orlin, the Ascended being who once fell in love with her. He has reincorporated with a fragile human form, sacrificing his immortality, to warn Earth that the Ori's promises of enlightenment are utterly false. In fact, the Ori demand belief only because they derive sustenance and power from the worship of the unsuspecting Priors and their other followers. Essentially, the Ori suck energy from their worshippers, and then abandon them to die. If the Ori convert Earth and the Milky Way, the energy of billions of new worshippers will make them stronger than ever. To find a cure for the plague, Orlin needs a blood sample from the Prior who originally gave the disease to SG-6 on P2X-885. Daniel and Mitchell must track down that Prior, then hope that Carter's untested weapon really can suppress a Prior's superhuman powers. If it doesn't, and they fail, Earth's entire population will die with them.