With Vala missing in action after the Kallana incident, Mitchell unites SG-1 under his command, and not a moment too soon. A Jaffa warrior belonging to the warlord Gerak turns up dead on a rural Virginia roadside, and the team has to figure out why.
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SG-1 travels to a medieval society called Camelot to find a device, made by Merlin, which can help them defeat the Ori. They investigate library guarded by a fierce hologram called the Black Knight as the threat of the invasion looms.
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With the departure of Gen. O’Neill, Stargate Command enters a new era. That said, the new commanding officer, Gen. Landry, and SG-1's new leader, Lt. Col. Mitchell, barely have time to settle in before the sudden return of the beautiful thief Vala.
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The plague continues to spread. Teal’c pleads with the Jaffa High Council to reject Origin, but the Ori have transformed Gerak into a Prior. Cameron and Daniel seek the blood of the Prior that caused the plague in order to cure it.
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When Carter starts a device made by Merlin, it transports her and Mitchell to a parallel dimension.. Meanwhile Teal’c and SG-12 travel to the planet where a whole Sodan village is exterminated by its own hero Volnek, turned evil by a Prior.
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Just before the Jaffa Nation holds its elections, two of the council’s devout supporters of democracy announce that they no longer condone the plan. Teal'c and Bra'tac conclude that treachery, not honest dissent, must be behind the strange behavior.
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A Ori Prior has established an ever-growing force-field on a Jaffa-controlled planet. A minor Goa’uld, but also the one who engineered the multiple gate connections required for the defeat of the Replicators, comes to the SGC to offer his help.
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An Ori priest saves Daniel and Vala from the fanatical Ori followers, but they’re still trapped in "borrowed" bodies in a distant galaxy. The priest transports them to the Ori's holy realm, where Daniel uncovers the secrets of the mysterious gods.
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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.
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A Prior has convinced Gerak, leader of the Jaffa Nation, into following Origin. Teal’c and Bra'tac attempt to make a case against the Ori, but to no avail. Meanwhile, a plague released by the Ori has breached SGC and spread to the rest of Colorado.
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After an hour’s separation from Daniel, Vala returns to SGC. Although the bracelets have been removed, their effects have not diminished. To break the link, Mitchell, Jackson, and Teal'c must deal with many people that Vala has used over the years.
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Vala’s speaks to Stargate Command through Daniel's body. The Ori had sent her through a black hole to their home galaxy, where she learned that they plan to invade the Milky Way through a supergate, with an unknown location in the Milky Way.
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