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In the opening scene, we see a different-looking Carter and the long-dead Charles Kawalsky fighting Jaffa in the SGC before stepping through the quantum mirror (as seen in "There but for the Grace of God") into a storage warehouse in our universe. They are promptly taken into custody, but are quickly brought to the SGC when they demonstrate extensive knowledge of the Stargate program.
This Carter and Kawalsky cannot be the ones Daniel encountered in season 1 in "There but for the Grace of God", since we saw those characters die in that episode. However, as the Carters explain, since there are an infinite number of universe representing the infinite choices each person makes, they are from a similar alternate universe. In their alternate universe, Apophis and his Jaffa were just starting to take over the SGC when they escaped. O'Neill, who was Carter's husband, had died, as had Daniel - in fact, neither Carter nor Kawalsky knew Daniel at all. In the alternate universe, Teal'c was still First Prime for Apophis and leading the attack against the SGC.
The alternate Carter (who I'll call "alt-Carter" for clarity) and Kawalsky request asylum in "our" universe at the SGC. They bring their alternate knowledge of the Goa'uld, and alt-Carter obviously has her genius to offer - even O'Neill comments on how lucky they'd be to have two Carters working for them. I find it a little strange that they'd abandon their own world so quickly - we are led to believe at first the the Earth was completely lost, but later we find out things were not quite so bad. At any rate, in very short order, the government approves the request for asylum.
While alt-Carter and Kawalsky are relieved, alt-Carter in particular is having a very hard time adjusting. She is still completely shattered by her O'Neill's death and has a hard time comprehending that our O'Neill is a completely different person. To his credit, O'Neill does try hard to offer her support, but he clearly doesn't really know what to do. Our Carter doesn't seem to be having as hard of a time adjusting to having a "duplicate" of herself around, but then again, that's the only change she has to deal with. Interestingly, neither of the Carters understands the other's decision to go into or not go into the military.
Any resolution to the situation is short-lived, because alt-Carter soon begins having some kind of weird seizures. The two Carters realize they are caused by "entropic cascade failure" - basically having two of the same person in one universe is causing the problem. Alt-Carter will die soon if she remains in our universe.
They could just send alt-Carter back through the quantum mirror - Kawalsky is safe because his counterpart is dead. However, that would be an obvious death sentence given the state of affairs on Earth in the alternate universe. Carter realizes that if alt-Carter can contact the Asgard in the alternate universe, they might be able to stop the Goa'uld attack on the Earth. They develop a plan to rebuild the power device that O'Neill used in "The Fifth Race" to dial the Asgard's Stargate in another galaxy, take the device into the alternate universe, and divert Apophis's attack long enough for Carter to contact the Asgard via the Stargate. O'Neill, Daniel, Kawalsky, and alt-Carter will all travel to the alternate universe. (Not Carter because they'd cause the corresponding entropic cascade failure in the alternate universe; somehow they don't worry about Teal'c.)
Surprisingly, General Hammond approves their plan. I say surprisingly, because I cannot fathom why he'd approve it. If the plan succeeds, yes, alt-Carter, Kawalsky, and the alternate Earth will survive. This has no benefit at all to our Earth, and in fact, Hammond makes it clear that he will prevent the quantum mirror from ever being used again after this. On the other hand, if something goes wrong with the plan, O'Neill and/or Daniel could die, which would be a huge blow to our Stargate program, and conceivably the quantum mirror could fall into the hands of the alternate Apophis. This seems like a huge negative, and given that Apophis already nearly has control of the alternate SGC and Earth, it seems more likely that the plan will fail than succeed. Hammond really should have sent alt-Carter back, possibly with the power device and Asgard Stargate address (and Kawalsky), but that's it. However, it appears that the sentimentality of everyone involved carried the day and thus the plan goes forward.
In amazingly short order, the Carters fix the power supply to dial the Asgard Stargate address. Kawalsky teaches Daniel how to find specific universes in the quantum mirror, and they are good to go.
The team enters the alternate universe, and right away things go wrong, when Daniel has to turn off the quantum mirror, thereby losing their "return address". Things improve, however, when Teal'c kills alt-Teal'c and takes his place. (I was amused to see that alt-Teal'c had a goatee, thereby following the "evil mirror universe" tradition set by Star Trek: The Original Series in "Mirror, Mirror".) He's able to order the Jaffa around and expedite the placement of the power device for the Stargate and the dialing of the Asgard's Stargate address. But just as alt-Carter goes through the Stargate to the Asgard galaxy, Apophis arrives and realizes Teal'c is betraying him.
Apophis and his minions spend some time threatening the good guys and torturing, then killing, the alternate universe Hammond. His pyramid ship is about to land on top of the SGC. Just when things look the worst, of course, the Asgard suddenly begin beaming away Apophis and all the Jaffa - although some escape through the Stargate - and return a revived Hammond.
Daniel has relocated our universe through the quantum mirror, and everyone but O'Neill returns to our universe. Alt-Carter and O'Neill share a kiss goodbye, and alt-Carter finally acknowledges that O'Neill is not really "her" O'Neill.
This is an exciting and fast-paced episode, and it's a guilty pleasure in a lot of ways: we get to see the SGC destroyed, but not really; we get to see O'Neill and Carter get together, but not really; we get to defeat Apophis again, even though he's dead in our universe. I always enjoy this episode a lot; my only problem with it is the motivate I mentioned above - there's really no reason our SGC should risk so much to help the alternate universe. I'm not convinced that a moral argument about helping those in need whenever possible even works here; since every choice makes a new universe, this just means that there is another new universe out there: the original, where we helped them, and the new second one, where we didn't. So what's the point?
Of course, if the many-universes hypothesis is true (that each choice creates a new universe), then one could say that all choices for good are futile, because there's always the alternate universe where the choice was not made for good and whatever bad outcome resulted from that still happened. If you think about this too much, it could lead to complete paralysis about what to do. I almost think you'd have to go with Teal'c's philosophy: no other universe are of consequence, just the one you're in right now. Of course, that doesn't seem morally right, either. It's a big can of worms that this episode opens.
This episode, like season 1's "There but for the Grace of God", highlights the idea that simple choices can have huge results. Which of the choices made by people in the alternate universe led to them being so unprepared for Apophis's assault? Daniel's not joining (or not being asked to join) the SGC? Carter not joining the military? SG-1 not asking Teal'c to betray Apophis? Teal'c refusing to do so? Or many smaller choices? Again, if one dwells on this too much - the idea that a simple choice could have huge ramifications - it could paralyze you into doing nothing. Of course, that's a choice, too. It's definitely interesting to see how the universes differed.
Of course, one of the main points of this episode was the fact that alt-Carter was married to the O'Neill in her universe. When she arrived in our universe, clearly she met a person, O'Neill, who looked like her husband and in some cases acted like her husband, but was not her husband. This brings up another interesting question: how much of a person's personality is inherent to them regardless of their choices and experiences, and how much develops because of those choices and experiences? Alt-Carter's O'Neill and our O'Neill obviously had very different experiences, at least in the years involving the SGC. How much did those different experiences make them different? Could our O'Neil have very quickly taken the place of alt-Carter's dead O'Neill? Could he have fallen in love with alt-Carter quickly and just fit right in? Or would his different experiences and choices have made him not quite "right" for that relationship? That's very hard to say, but it's an interesting question.
Of course, what this plot point did introduce (or perhaps follow up on from "There but for the Grace of God") was the idea of a Carter/O'Neill romance. In "There but for the Grace of God", only Daniel "experienced" the relationship between the alternate universe Carter and O'Neill. I'm sure he reported back on it in his de-brief, but it would've been a dry fact. In this episode, our Carter and O'Neill experienced that relationship first hand, so to speak. Has it given them any ideas? Certainly it changed O'Neill's perspective a bit, as he was genuinely ready to listen to Carter discuss her feelings about having her alternate be present, whereas Carter had no intention of telling him.
I'm of mixed feelings about them getting together. While a romance can be fun, especially one that is "forbidden", it's really nice to see a platonic, friendship relationship between a man and woman in a TV show. So I don't know that I care for this to go anywhere.