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At the SGC, the Goa'uld Nerus has initiated contact, wanting to work with the SGC against the Ori. He had previously been one of Baal's minion and claims he was the one who figured out how to simultaneously dial all stargates in the galaxy in last season's "Reckoning, Part 2". From his appearance and "dietary requirements", Nerus could have inspired the figure of the God of Gluttony. Despite his appearance, he is quite intelligent. He has reconnaissance satellites orbiting Kallana, and updates the SGC on the Prior's activities there: the forcefield is growing, and the stargate has been open for much longer than its usual 38 minute limit. He encourages the SGC to attack the Ori there as quickly as possible and with as much force as possible.
The SGC does realize that the Prior's control of the stargate and the planet is a possible beachhead for invasion. They plan an attack, using a naquaadria-enhanced nuclear warhead known as a "gatebuster", specifically designed to destroy a stargate. Lt. Col. Samantha Carter is the lead researcher on this weapon, so she returns to the SGC to join the mission. SG-1 (Mitchell, Carter, Teal'c, Daniel, and Vala) are taken to Kallana by the Prometheus.
When they arrive at Kallana, they beam down to the surface with the warhead - they can beam through the forcefield when it is in an "expansion phase". It's really nifty under the forcefield: it is opaque to light, so it's dark underneath. The only thing there is the Prior. Their conversation is blackly humorous: both sides know they are not going to change the opinion of the other side, so they deliver their statements in a very peremptory manner. Mitchell gives the Prior 30 minutes to get rid of the forcefield and leave, or the bomb will go off.
Meanwhile, Gerak arrives at the planet with a number of Jaffa ships. They begin attacking the forcefield; once SG-1 is beamed back to the Prometheus, Carter realizes that the forcefield is growing from the energy of the weapons fired at it, so the Jaffa attack is accelerating its growth. They manage to get the Jaffa to cease fire until the bomb explodes: when it does, we see everything inside the forcefield become red-hot vapor, but the forcefield continues to grow. It covers the entire planet.
Somehow the stargate was not destroyed, and a number of objects come through it and head away from the planet. They incidentally collide with and destroy some Jaffa ships. The Jaffa ships are forced to withdraw, but leave behind a small cloaked ship to observe further developments. The unidentified objects begin organizing into a large ring shape: a supergate, a huge stargate. The forcefield has started to force the planet to shrink into a singularity - the energy from the collapse is providing power to keep the forcefield going and the stargate open.
If the supergate finishes forming, the Ori will presumably have an open door into our galaxy that is large enough to bring through many very large ships. Vala hijacks the cloaked Jaffa ship and uses it to take the place of one of the supergate panels. When the planet finishes collapsing and the energy is channeled into the supergate, the supergate overloads because of the faulty "panel" and is destroyed. We see Vala ring away from the ship, but she does not reappear on the Prometheus; Carter speculates that her signal was sent through the singularity to singularity in the Ori galaxy, where she may have been rematerialized.
SG-1 realizes that Nerus led them into the attack on the forcefield because he knew it would make it grow faster. When confronted by Landry, Nerus reveals that he has converted to Origin in order to ascend when he died. He expects that the SGC will kill him for his deceit, so he is looking forward to his ascension. However, Landry says they plan to keep him imprisoned, and on a restricted diet, until he decides to help them. A fitting end!
Although the SGC managed to win a victory over the Ori in this episode, it also really highlighted what intelligent and effective opponents the Ori are. They clearly have a good grasp of human psychology, taking advantage of the instinct of the humans and Jaffa to attack an enemy and using that attack to power their own devices. They also apparently are aware of the power struggles in the Milky Way galaxy and are willing to enlist Goa'uld as allies when it suits them. They also seem to have some big plans: the supergate would have been capable of transporting some very large objects. The Ori don't do anything part way!
Nerus was also a clever enemy, although he turned out to be too clever for his own good in the end when he didn't realize he wouldn't be executed. I know that Landry, Mitchell, and others said that they didn't actually trust Nerus, but they still seemed to fall for him hook, line, and sinker. Was I the only one picturing O'Neill shouting, "Don't trust him! He's a Goa'uld!" I really think O'Neill would have somehow anticipated the nature of Nerus's betrayal. Ah, probably wishful thinking. I loved Landry telling Nerus that chicken was a rare Earth delicacy.
The naquaadria-enhanced nuclear warhead is a scary weapon. Even though it didn't work in this case, it is clearly very destructive. The effects of it under the forcefield dome were impressive, and I liked the mention that everything there had been turned into a superheated plasma, which would be completely correct. The reactions of some of the characters to this weapon were interesting. Mitchell, although he is career military, definitely did not even like the idea of the weapon, let along using it. Carter, however, was apparently one of its key architects - would she really work on such a device that can obviously only be used for destruction?
The SGC and the Jaffa under Gerak still can't seem to get along. Gerak seems to be intentionally interpreting every act by SGC personnel in the worst possible light. I feel like he's just looking for some excuse to start a fight with the SGC, although that would be a foolish thing to do. Will SG-1's destruction of the supergate improve their standing with him? Probably not, when a Jaffa ship was hijacked to do it. In any case, Gerak needs to realize that shooting first and asking questions later is not the best course of action in all cases.
Finally, there's Vala. While I often find Vala annoying, she was rudely told to shut up and ridiculed way too much in most of this episode. To some extent, this was just setting her up for the end of the episode, but it was grating in itself. I generally like the SG-1 characters, so I didn't like seeing them be so boorish. They know that Vala's not stupid, so why act like she just doesn't exist? I did enjoy her exchange with Daniel when they boarded the Prometheus: Daniel said something like, "You do know this is a military ship, right?" and she replied, "Yes, Daniel, I stole it." Ha.
Vala's actions at the end certainly belie the image of herself that she projects: that she is only concerned for herself. Certainly, she would eventually suffer if the Ori took over the galaxy, but until they did she would get along well enough, so her actions could not be strictly selfish. Hopefully she did survive and she is now giving the Ori hell.
The idea of using the energy from an object collapsing into a singularity is a very good one, and relatively realistic as far as science fiction ideas go. The amount of energy released when a planet-sized object collapses into a singularity is about the same amount of energy released when a dying high-mss star explodes in a supernova: a lot! The astronomy textbook my students use estimates this to be about 10^24 times more energy than the annual energy consumption in the US and 10^30 times more energy than a 1 megaton hydrogen bomb. Of course, making a planet-sized object actually collapse would take some energy itself - how did the Prior do this? The Priors clearly have some awesome technology at their command.
How long until the Ori try to establish another beachhead in our galaxy? How will the SGC be able to monitor everywhere to find out where their incursions are?