Local Jaffa on a desert planet approach the Stargate and the Prior positioned there, and tell him to get lost. He doesn't take it well, his staff glows, and the Jaffa open fire. His personal force field protects them and then he sends them flying miles away and generates a force dome around himself.
At SGC, they get an unauthorized activation and a text message – in Goa'uld. Vala, Teal'c, Daniel and Mitchell are playing basketball and Landry asks Teal'c for information on the message. It's from Nerus, a Goa'uld known for his appetites and his scientific knowledge, and he asks for a meeting with SGC. They're interrupted in their briefing by an off-world activation and Nerus comes through after going through a security check. Nerus claims to be impressed by Teal'c and Daniel as the ones who have put the era of false gods to an end.
The Jaffa are still trying to break through the force dome surrounding the Prior and the Stargate, while Nerus chows down. Nerus demonstrates a holo-projector that shows a forcefield surrounding the planet Kalanna and notes it is expanding. Worse, the Stargate hasn't closed down since the Prior arrived days earlier. Nerus volunteers his services to aid them and they agree after he points out what has happened is an "invasion", and that they are, to some degree, responsible.
Landry is talking to Jack trying to get Carter to show up, unaware she's already arrived for a temporary assignment. They're trying to open the Stargate but can't make the connection past the open wormhole. Sam reveals they've developed something at Area 51 but she's not sure if it'll be enough. They use the Prometheus to head for the planet and Vala has to come along, as Daniel is concerned the link between them might still exist. They reveal the weapon is a "Gatebuster" as the Prometheus goes into hyperdrive. Nerus remains at SGC, chowing down as Landry suggests he might help a bit more – Nerus warns him that force will be necessary.
The Prometheus arrives and detects the environment inside the force field is still toxic, but the Prior is fine. Donning space suits, the team (except for Sam) beams down during an expansion phase of the field with the Gatebuster, to talk to the Prior. He isn't impressed and they pass on the word to the Ori that the bomb will go off if tampered with, the Prior threatens them if they don't withdraw. The Prior is unimpressed and unconcerned whilst the team waits for another expansion to beam out, but then a fleet of Jaffa ships comes out of hyperspace and opens fire on the force field. The energy fire causes an expansion, they activate the Gatebuster, and beam out while the Prior remains, confident.
Sam has to deal with Gerak, who is leading the fleet and is unimpressed with their efforts so far. The team can't tell him they learned about him from a Goa'uld. On the planet, the Prior does something with the Gate and the bomb detonates. In response the force field expands, feeding on the energy and begins covering the entire planet. Gerak insists SGC attacks with him, or the Prometheus will be his next target. The Jaffa fleet opens fire and Mitchell orders they open fire as well in the hope they can collapse the weakened field. The force field loses strength but Sam realizes the field is feeding off the energy weapons – the Ori planned it, and Nerus set it up. Cameron orders a cease-fire but before they can talk to Gerak, the planet is totally encompassed. Multiple objects come out of the Stargate and impact the fleet. The Prometheus is damaged and forced to retreat. At SGC, Landry confronts Nerus, who reveals he's working with the Ori and boasts of his tricking them into powering the beachhead.
Gerak is not happy – two of his ships have been damaged and he withdraws – the planet is collapsing within the force field and there's no reason for him to stay. Sam confirms the collapse and picks up a new signal – the objects that came through are forming a vast super-sized Stargate. Sam realizes the Ori are collapsing the planet into a black hole to power the energy at this end – there's one at the other end as well and if the Ori succeed in forming the second black hole they will have a permanent gate.
When the team ignores Vala to come up with a plan, she heads out with her own plan. She ring-transports over to a cloaked cargo ship that Gerak left behind, knocks out the Jaffa, and takes the ship toward the super-gate to break the "chain." She inserts the cargo ship into the last slot in the chain in place of the incoming object, disrupting the energy flow and causing an explosive chain reaction. The singularity of the planet starts to form as Vala tries to ring-transport out. The black hole forms but the gate is destroyed, but Vala didn't succeed in transporting out. Daniel starts to collapse due to their leftover link and the impact of her death. Landry confronts Nerus and lets him know what happened, then has him locked up and puts him to work on a defense against the Ori.
Mitchell tries to convince Sam to join the team as they meet with a recovering Daniel, and Sam lets him know that her transporter beam might have been pulled into the singularity and be somewhere in the Ori home galaxy.