Beachhead

Beachhead is the sixth episode of the ninth season of Stargate SG-1.

Plot
When Nerus, a minor Goa'uld and hedonist, requests an alliance with Earth, General Landry and SG-1 don't need Vala to warn them that Nerus can't be trusted. Vala warns them anyway, but Landry decides to risk inviting Nerus to Stargate Command.

Nerus explains that a mysterious force field has appeared on the planet Kallana. The field surrounds Kallana's Stargate and periodically increases in size. Worse, the Stargate hasn't shut down for two days, which violates all known laws of physics. Worst of all, there's a Prior of the Ori on the scene. The team is forced to agree with Nerus's frightening conclusion: The Ori must be establishing a beachhead from which to launch their invasion of our galaxy.

Landry orders Mitchell and his team to use any means necessary to stop the Ori. To that end, he sends along the military's latest superweapon, the Mark IX "Gatebuster" naquadria-enhanced nuclear warhead. He also sends the Mark IX's designer: Lt. Colonel Samantha Carter. SG-1's pleasure at working together again is tarnished only slightly by Vala tagging along. Her life is still linked to Daniel's, but now, in the presence of the full SG-1 team, even she can tell that she's just a fifth wheel.

SG-1 travels aboard the Prometheus to Kallana, where the force field has grown so large that it's visible from orbit. Mitchell, Daniel, Teal'c, and Vala manage to transport down to the planet's surface with the Mark IX. They try to negotiate with the Prior, threatening to use the Mark IX in 30 “earth minutes”, but even the threat of imminent nuclear vaporization doesn't budge him.

Suddenly, a bombardment of energy weapons rains down from above. Gerak, the leader of the free Jaffa, has learned of the Ori incursion and demands to take matters into his own hands. At the same time, the force field expands again. Out of options, Mitchell arms the bomb and transports his team back to Prometheus.

Humans and Jaffa alike brace for impact as the Mark IX detonates below. As the shockwave dissipates, they are stunned to discover that the force field has only gotten bigger. Soon it will engulf the entire planet, and they have no idea what will happen next. Gerak resolves to continue his bombardment of the surface, threatening to target the Prometheus once he has dispatched the Ori if the Tau'ri do not join him in his attack.

Mitchell gives the order to attack. Before long, Carter notes that their attacks are only making the forcefield bigger: The force field is using their attacks to power and increase itself, and as Nerus, who reveals he has been working with the Ori all along, informs General Landry, the Ori would have been unable to get this far without Earth's Mark IX. The Tau'ri plan has backfired.

Once the force field has engulfed the entire planet, a number of small, rectangular projectiles eject from the Stargate, destroying two of Gerak's ships. Meanwhile, the force field begins 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 gapes in awe as the first Supergate forms before their eyes. The Supergate, between 300 and 400 meters in radius, powered by a black hole, is large enough to allow an Ori armada to enter the Milky Way galaxy.

Carter notices that the pieces of the Supergate are becoming interconnected by a common energy signature. Vala, who realizes that Gerak has left behind a cloaked cargo ship to watch them, uses the transporter rings to board the ship. She moves the ship into the path of the last piece of the Supergate, disrupting its energy signature. When the planet finally collapses and the micro-singularity forms, the shockwave destroys the cargo ship and disperses the now-inert pieces of the Supergate. The Ori beachhead is thus destroyed, and Nerus is sent to a prison underground in Area 51 until he comes up with a plan to fight the Ori. Nerus asks, "What would possibly compel me to do that?" General Landry answers, "Hunger."

Vala vanishes after the explosion, which puts Daniel in danger, but SG-1 soon realizes that the effect of the bracelets has begun to wear off. Carter notices that a matter stream left the cargo ship before it exploded. Vala may still be alive somewhere in the Ori galaxy. Daniel breathes a sigh of relief, saying, "Well, we wanted to send them a message."

Quotes
Cameron Mitchell: Oh boy, and here I was trying to have a nuke-free career.