SG-1 are on a planet looking for Merlin's weapon when they find a medieval-style village. In the center of the village they spot a sword in a stone. The village governor, Meurik, greets them and welcomes them to…Camelot.
At a meeting they are introduced to the village historian, Antonius, and told the village is the future site of Arthur's arrival, but here it is believed Arthur defeated Mordred and then sought the Holy Grail. Daniel asks about Merlin and the villagers find the question upsetting. Antonius reveals that the villagers are leery of Merlin and his library remains sealed and protected by a powerful curse.
Mitchell is confident they can handle it but Daniel believes it may be tougher then the knight they fought in the Avalon cave. They split up and Mitchell demonstrates swordfighting to some children, and they reluctantly show them Merlin's library. The door is magicked and the girl, Valencia, tells them of a key in the village archives. Antonius refuses to help them out until Daniel appeals to his thirst for knowledge. Antonius secretly takes them to Merlin's library and lets them in. Inside they find a dusty library and start going through the books. Mitchell finds a secret passage and Antonius leaves rather then risk further danger. As he returns to his home he is confronted by a Black Knight, who approaches him.
In the secret chamber, Daniel is working to interpret the symbols when they hear a commotion and go to find that Antonius is dead. Meurik confronts them over their entry into the library and they realize the Knight is going after the villagers rather than appearing in the Ancient chamber like at the Avalon cave. Meurik doesn't believe their stories of the Ori and tells them to leave and take the curse with them. Daniel figures the curse indicates there is a solution and there should be a cutoff switch in the Ancient chamber. He plans to convince Meurik to reconsider and the other s reluctantly agree. Mitchell examines the sword and talks with Valencia, and Meurik confronts them again and tells them they'll be escorted to the Stargate. they are beamed up to the Odyssey by Colonel Emerson. who informs them that they've located a fully-formed Ori supergate. The Odyssey is heading toward the supergate to meet with as many Jaffa and Tok'ra ships as possible. Daniel suggests he stay behind and Mitchell volunteers to stay with him. Emerson agrees to beam them down and have the Russian ship recover them later.
Back in the library, Daniel goes through the books while Teal'c takes a separate ship to seek out some other allies. Daniel finds what he thinks is an access code and they step on to the platform which triggered the Black Knight the last time. Daniel enters the sequence and nothing happens…until they hear a scream from outside. Mitchell goes outside and confronts the Black Knight, which is immune to bullets. Mitchell takes a hit and is unable to lay a bow on his holographic opponent, and the villagers refuse to help him. Valencia goes for the stone and pulls it free, then tosses it to Mitchell. He's able to hold it off while Daniel enters a combination which causes a pre-programmed Merlin image to appear along with the treasure.
As Mitchell starts to go down, Daniel opens a chamber with the control crystals and shoots them, ending the Merlin and Black Knight projections simultaneously. The Odyssey meets with the other ships near the Ori supergate, where there's no signs of activity. They're unable to destroy the supergate so far.
Daniel figures out there's a red ruby pendant that has never appeared in any other references, and that the hologram Merlin was referring to it. While they search the library, another vessel meets with the fleet – it's the Asgard Ksavir, ready to lend his aid. Teal'c is brought before the Lucian Alliance, and asks for their assistance against the Ori. When they order his death, Teal'c knocks out his guards and then repeats his offer more emphatically.
Kvasir thinks the Asgard know what the anti-Ori weapon is – an energy-transfer weapon that will feed interference energy into the Ascended beings' higher dimension. If it can be sent to the Ori galaxy, it will destroy them. They conclude they must dial out the supergate before the Ori dial in, and send the pendant through.
Daniel and Mitchell aren't having much luck finding the pendant. Meurik arrives with the townspeople and Daniel asks him for information on the pendant. Meurik tells him that the pendant is the "holy grail," the Bloodstone that Arthur sought on his quest. Meurik gives them the names of three planets which Daniel has never heard of. Meurik believes that Arthur will return now and offers to let them stay, but they get beamed up again (this time to the Korolev, commanded by Colonel Chekov) before they can get any further.
Sam locates a dialing control crystal and they've reprogrammed a new one, but they don't have the pendant. Sam suggests they beam out anyway, open the gate out, and maintain it constantly via the singularity so that the Ori cant dial in. they beam Sam out where she magnetically locks onto the unit and starts to work. Before she can complete the installation an energy surge goes through the gate, eliminating Sam's magnetic lock and she drifts into space. They can't beam Sam out or establish contact due to the interference. As Sam looks on, the supergate opens and a huge wormhole forms.
Chekov's ship the Korolev arrives just as four massive Ori ships come through the gate. The combined fleet opens fire without much effect, and the Ori fire back destroying several ships. The Odyssey and Korolev lose power and Mitchell goes off to get the engines on-line while Chekov continues the fight. Three Lucian Alliance ships arrive, opening fire and buying them a temporary respite. The ships are bursting into flames and crew are dying by the hundreds. Sam can only look on at the carnage as the Ori ships continue on their way. And aboard one Ori ship, Vala looks on…and she feels a contraction from the baby she carries.