A Prior of the Ori came to the planet Kallana to convert its inhabitants to Origin. But the Jaffa of the planet, having previously worshiped the Goa'uld refused to follow the teachings of the Book of Origin and they were struck down by the Prior, resulting in their deaths. Soon afterwards, a mysterious blue forcefield appeared around the Prior and the Stargate. The Goa'uld Nerus contacted Stargate Command and informed them of the situation and that when the forcefield expands it weakens enough to be penetrated with Asgard transporters. (SG1: "Beachhead")
Major General Henry Landry ordered Lt. Colonel Cameron Mitchell and SG-1 to use any means necessary to stop the Ori. To that end, he sent along the military's latest superweapon, the Mark IX "Gatebuster" Naquadria-enhanced nuclear warhead. He also sent the Mark IX's designer: Lt. Colonel Samantha Carter.
SG-1 traveled aboard the Prometheus to Kallana, where the forcefield has grown so large that it's visible from orbit. Mitchell, Dr. Daniel Jackson, Teal'c, and Vala Mal Doran managed to transport down to the planet's surface with the Mark IX. They tried to negotiate with the Prior, threatening to use the Mark IX in 30 "earth minutes", but even the threat of imminent nuclear vaporization didn't budge him.
Suddenly, a bombardment of energy weapons rained down from above. Gerak, the leader of the Free Jaffa Nation, had learned of the Ori incursion and demanded to take matters into his own hands. At the same time, the forcefield expanded again. Out of options, Mitchell armed the bomb and transported his team back to Prometheus.
Humans and Jaffa alike braced for impact as the Mark IX detonated below. As the shockwave dissipated, they were stunned to discover that the forcefield had only gotten bigger. Gerak resolves to continue his bombardment of the surface, threatening to target the Prometheus once he had dispatched the Ori if the Tau'ri did not join him in his attack.
Mitchell gave the order to attack. Before long, Carter noted that their attacks were only making the forcefield bigger: The force field was using their attacks to power and increase itself, and as Nerus, who revealed he had been working with the Ori all along, informed Landry, the Ori would have been unable to get this far without Earth's Mark IX. The Tau'ri plan had backfired.
Once the forcefield had engulfed the entire planet, a number of small, rectangular projectiles ejected from the Stargate, destroying two of Gerak's Ha'taks. Meanwhile, the forcefield began to collapse the entire planet into a micro-singularity. Gerak retreats as the projectiles come together, forming a colossal ring in planetary orbit. SG-1 gaped in awe as the first Supergate formed before their eyes. The Supergate, between 300 and 400 meters in diameter, powered by a black hole, was large enough to allow an Ori fleet to enter the Milky Way galaxy.
Carter noticed that the pieces of the Supergate were becoming interconnected by a common energy signature. Vala, who realized that Gerak had left behind a cloaked Tel'tak to watch them, used the Transportation rings to board the ship. She moved the ship into the path of the last piece of the Supergate, disrupting its energy signature. When the planet finally collapsed and the micro-singularity formed, the shockwave destroyed the Tel'tak and dispersed the now-inert pieces of the Supergate. The Ori beachhead was thus destroyed, and Nerus was sent to a prison underground in Area 51 until he could come up with a plan to fight the Ori. (SG1: "Beachhead")