A large asteroid is discovered on a collision course with Earth.

Upon finding Merlin's testing grounds, Dr. Jackson, Teal'c, Col. Mitchell, and Vala uncover the hidden treasure including a book that tells the story of the Ancients coming to Earth from another galaxy and a strange alien device.

Daniel and Vala have used the Ancient device to inhabit the bodies of people in the Alterans' home galaxy. They have been taken by a Prior of the Ori to meet the Doci, one who speaks with the Ori, their gods. Daniel surmises that the Ori are what the Alterans who became the Ancients left behind and have ascended in a similar fashion, but without the restrictions.

Vala leads SG-1 to a planet which the Priors have visited with a plan to keep them from worshiping the Ori. Once there she reveals that the symbiote that once held her ruled over this planet. She feels that keeping them in allegiance to her will hinder the spread of Origin to this world.

The Ori unleash a deadly plague on Earth, prompting a powerful ally from SG-1's past to come to their aid.

When an entire people may suffer the fatal wrath of a Prior of the Ori for not converting, General Landry orders Carter to try out an experimental device based on Merlin's technology to transport their whole village. The test works, but before they can activate it on a sufficiently large scale, an Ori force takes the village, wounds Sam and kills villagers near the Stargate before Vala's very eyes. Her husband, Tomin, captures her but claims to not love her. He is ordered, as punishment for being deceived by her, to re-educate her. Sam and Cam are able to activate the functioning part of the device, so their building is cloaked from the Ori, who threaten the village leaders and discover and arrest Teal'c. Even after the remaining villagers bow in submission, the Prior decides to destroy everyone since he couldn't find Sam, which makes Tomin doubt his faith, inform Vala, and confront the Prior. The villagers want to sacrifice Teal'c, but one leader stops them. Having seen the Prior destroy the village, Tomin sends Vala down, where she sees if Carter had managed, just in time, to get the device working...