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Carter has been trying to figure out how one of the Ancient devices found under Glastonbury ("Avalon, Part 2") works. Mitchell comes to drag her away to breakfast, when she makes a breakthrough. She uses her computer to activate something on the device, and there is a bright flash of light. Afterward, nothing seems to have changed. However, she and Mitchell quickly discover that no one can see or hear them, and that other objects pass through them.
Carter hypothesizes that they have been transported to an alternate dimension. As she says, Daniel suffered the same effects as they have in season 3's "Crystal Skull"; in that episode, Daniel could be seen by his grandfather because his grandfather had previously had the same experience. So Carter's first idea is to go see if Daniel can see them. He can't. Apparently they are in an alternate alternate dimension. While Carter is correct that some hypotheses in physics (mostly those trying to unite the theories of general relativity and quantum mechanics) do postulate addition spatial dimension to the three we experience in everyday life, the way the alternate dimension is presented here has definite problems. For example, why do Carter and Mitchell not fall through the floor? How can they breathe, when all the air is "real" air? I have a much longer discussion of my problems with Stargate SG-1's use of the "alternate phase" (as this alternate dimension will be called) in my review of season 10's "Line in the Sand", so I won't get into it a lot here.
Daniel and Dr. Lee do some very clever work with the security cameras to prove that Carter and Mitchell disappeared from the room and didn't leave. When Dr. Lee mentions that there is residual lepton radiation, Daniel remembers that from his own alternate dimension experience. He does some research on the Ancients, Merlin, and Merlin's research, and discovers that Merlin had created a device called "Arthur's mantle" which rendered the user invisible. Daniel believes the invisibility was effected by transporting the user to an alternate dimension.
With this breakthrough, Daniel and the others realize that Carter and Mitchell are still there, just invisible. Carter and Mitchell discover that the only physical object they can move are the keys on Merlin's device. When they touch it, a heads-up display (HUD) activates, but it's only invisible in the alternate dimension. Daniel and Dr. Lee notice the keys moving, and they quickly set up a rudimentary communication scheme with Carter and Mitchell. The HUD may explain how the device works, but it is in Ancient. A laborious translation process between Daniel and Carter and Mitchell begins.
Meanwhile, the SGC had received a radio message through the stargate, but it was severely jumbled, so it took awhile to figure it out. It was from the Sodan: they were under some kind of attack - we can even hear staff weapons firing. General Landry send Teal'c and SG-12 to the Sodan's planet to check it out.
Teal'c and SG-12 find the Sodan completely wiped out, except for their leader, Haikon, who is critically injured. Haikon tells Teal'c that Volnek (the injured Sodan who was nursed back to health by the SGC in "Babylon") is to blame for the massacre. Volnek had encountered a Prior on another world and was "cursed" by him. After a few days, Volnek became ill (almost rabid, as we see flashbacks of him foaming at the mouth), and then transformed into some type of animal/human creature. Apparently his only thought is to kill, which he did ruthlessly. In addition, Volnek still has use of the Sodan invisibility device, making him even more dangerous.
Teal'c tries to track down Volnek, using another one of the invisibility devices, but only manages to temporarily wound him. Volnek apparently can now heal quickly, like we saw the first Unas do in season 1's "Thor's Hammer". Teal'c is wounded. Since Teal'c has not reported back to the SGC, SG-3 is sent to help. Mitchell accompanies SG-3, even though he cannot affect anything in the real world. Teal'c and Col. Reynolds of SG-3 devise a plan. Teal'c will lead Volnek into a trap: two claymore mines with a tripwire. The mines should tear Volnek's body into enough pieces so that he cannot regenerate himself.
Back at the SGC, Daniel is making slow progress on the translation. Dr. Lee is trying to recreate what Carter did to cause the initial incident. He thinks he found a way to reverse it, but he ends up sending Daniel to the alternate dimension, too. This turns out to be helpful, since Daniel can now read the device's display directly. The device was built by Merlin to hide some work from the other ascended Ancients. What was he hiding? A weapon that can kill ascended beings. Oh, and there's a gate address.
On the Sodan world, Teal'c turns on his invisibility device to lure Volnek to the trap. When the device is on, he can see and hear Mitchell, although Mitchell is still not solid to him. This makes some sense, because the invisibility devices were given to the Sodan by the Ancients; presumably they are some alteration of Merlin's device, or vice versa. Mitchell volunteers to lure Volnek, since he can't be hurt.
At the SGC, Dr. Lee inadvertently causes the device to lose most of its power. In desperation, Daniel manages to figure out the key commands to change them back to "reality". They don't know what will happen to Mitchell, but Carter suspects the device is capable of sending him back, too. They return safely.
Of course, they activate the device just at the moment Mitchell is about to lead Volnek into the trap. Suddenly, he becomes very vulnerable to the bestial Volnek, but Teal'c manages to rescue Mitchell, and together they goad Vonek into the trap, where he is killed. They return to the SGC.
This episode was fun to watch, mostly because of the character interaction. Mitchell's initial concern at being in the alternate dimension is that he's missing breakfast and lunch (roast beef!). Dr. Lee perpetually doing the wrong thing; he really can't be that incompetent, or he wouldn't be one of the top scientists at the SGC! His initial idea that Carter and Mitchell might have been miniaturized (Honey, I shrunk SG-1!) was a hoot, though. Then when Daniel is accidentally put into the alternate dimension, he just gives a good-natured shrug.
The two plots underpinning the show were seriously lacking, however. I already mentioned the problems with how the alternate dimension is presented. Watching the characters in the alternate dimension is really nothing new, since we saw it all with Daniel in "Crystal Skull". I didn't have as much of an issue with the "alternate dimension" idea in that episode, because it was basically a scenario where Daniel had to be found and rescued, and that was it. But now, Merlin's device is obviously being worked in as an item that could be used in the future. That will just compound all of the problems!
The second plot line, with the Sodan, is also problematic. It does make sense that the Priors would want to punish the Sodan for rejecting the Ori. The means of that punishment do not seem logical, however. Why alter Volnek to kill everyone? First, how did the Prior know that he would succeed? Second, why would they want to kill everyone? That deprives the Ori of potential worshipers. The Prior potentially could have stepped in and "saved" some of the Sodan from Volnek, but he did not. Given how influential the Sodan are among the Jaffa, one would think the Priors would be trying more to convert them, not kill them. Are the Priors using this kind of attack on other worlds? Given this type of attack and the flesh-eating bugs from "The Scourge", have the Priors stopped trying to convert people and instead just decided to kill them? In the end, it's a shame to wipe out the Sodan, since they were such a different faction of Jaffa.
Teal'c has to win some kind of prize for the best poker face: when he turns on his invisibility device and can suddenly see Mitchell, he gives absolutely no indication! Anyone else would surely be startled into some type of reaction, since Mitchell wasn't even known to be on that planet.
What are the long-term effects of this episode? The Sodan are no more. The Priors have shown another insidious method of attack. The SGC has a nifty new device, which they doubtless will employ extensively in the future, more's the pity. However, their use will be hampered initially by the fact that the device is very low on power.