While investigating the source of an addictive corn-like crop, SG-1 becomes involved in a firefight. They are forced to fall back to the gate, but as Daniel is dialing both the Stargate and the DHD are beamed away.

Based on the information Vala gave them, SG-1 travels to the planet where Merlin may have hidden his secret weapon to defeat ascended beings. There they find an appropriately medieval village and learn that Merlin kept a library there. The locals have never been inside owing to a curse on anyone who dares enter, but SG-1 finds a way in. Once inside, they find a secret chamber that may reveal the location of the weapon. The SGC, meanwhile, has located the Ori's Supergate. Sam's attempt to get control of it is too late, and four Ori ships make their way through. In the ensuing battle, much of Earth's fleet is destroyed, and the future of several SGC members is left in doubt.

As the Ori invasion continues, Vala and Daniel must deal with their leader, Vala's young daughter, who is rapidly aged by the Ori to serve their purposes.

While Daniel and Vala search for Merlin's anti-Ori weapon in Atlantis' library, Mitchell, McKay and Carter try to to dial the Ori Supergate in the Milky Way.

SG-1 is caught in the middle of a war after the Jaffa use a powerful weapon to kill 10,000 Ori followers.

Cameron Mitchell must go undercover inside the Lucian Alliance to find the location of the hijacked Odyssey.