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.

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

SG-1 blows up three transport ships in a row; Netan puts a price on their heads. General Landry grants the team members spare time on Earth. Daniel spends it researching in a library, where he meets a bounty hunter who tries to seduce him, then switches to her handgun... but is taken out by a bus. Sam gives a lecture on Air Force technological progress together with Dr. Bill Lee; a sniper nearly misses, but is taken out with an experimental weapon they'd brought along to demonstrate. Teal'c is injured by surprise at a Jaffa camp and successfully traps the killer to try again. Cam goes to his Kansas high school reunion and Vala succeeds in making him bring her along, pretending to be his accountant and partner. His still sexy former flame he'd believed out of his reach, Amy Vandenberg, is initially scared off by Vala, but, divorced and interested, explains she still has a secret crush on him. Alien bounty hunter Odai Ventrell assumes numerous classmates' appearances to kill him. Vala's failed rescue gets them both captured publicly, so he threatens to kill off reunion guests unless the other SG-1 members take their place, but they trick him using Chimera technology; they hope Netan's failure to eliminate them will rekindle the power struggle and Odai reacts surprisingly.