The threat of an imminent Ori invasion hangs heavily over the galaxy as SG-1 searches a planet where Merlin may have hidden a weapon capable of destroying Ascended beings. In a village with the auspicious name of Camelot, Meurik, the governor, and Antonius, a historian, explain that Arthur and his knights departed on a quest long ago and never returned. Although Merlin vanished then, too, his library remains - but the villagers believe that it's cursed, so no one dares to go inside. Antonius nervously helps SG-1 enter the forbidden building. Although the team is braced for booby traps, they find only piles of manuscripts and a secret passageway that conceals an Ancient control panel. Antonius isn't so lucky: A holographic Black Knight kills him after he hurries home. Apparently, the Black Knight is programmed to terrorize the village whenever anyone enters the library. Worse, before SG-1 can de-activate the murderous hologram, the Odyssey unexpectedly beams them away. They're needed elsewhere, because the Jaffa have discovered the Ori's supergate. A coalition of human, Jaffa, Asgard and Tok'ra warships are assembling there, ready for battle. Because Merlin's weapon could stop the Ori in a single stroke, however, SG-1 splits up. Daniel and Mitchell beam back to the library; the new Russian ship Korolev will pick them up later on its own trip to the supergate. Carter leaves with the Odyssey, and Teal'c seeks out the renegade Lucian Alliance to persuade them to join the fight. In Camelot, the Black Knight nearly kills Mitchell before Daniel manages to deactivate it. Afterward, Daniel pieces together clues that suggest the weapon looks like a red stone pendant, but it isn't in the library. Instead, according to Meurik, the missing stone was the object of Arthur's quest. Before Daniel and Mitchell can figure out how to track down Arthur, however, the Korolev beams them away. Meanwhile, the supergate opens and the massive Ori ships unleash their weapons. The holy war has begun.