This is for me one of the most fascinating and gut-wrenching episodes of the series.
IMDb asks if this review contains spoilers - and I am not sure it is possible to fill the required character limit for a review without spoiling something, in the case of this particular episode. I will leave out how it ends, in order to justify me selecting "No". The most basic of any kind of useful summary one can make is that in an alternate universe, Carter and Kawalsky under fire from Goa'uld invaders who have all but conquered Earth, make a desperate escape into "our" reality. The SGC in "our" universe now has to deal with the presence of alternate variations of individuals involved with the Stargate program, one of whom still is alive.
In the alternate universe, Sam is a civilian doctor of science, who was also married to O'Neill. The O'Neill in her universe had recently been KIA in the war against the Goa'uld, and she has to come to grips with the fact that the man she sees in the new universe and thinks she knows does not see her the same way. Only Amanda Tapping could know, but I imagine this was one of the most challenging episodes to record for her, given the emotional maelstrom the Carter from the alternative reality is caught in. It is one of the few episodes of the series in which I found myself choking back tears on.
It's not all tragedy though, or else it wouldn't be 'Stargate SG-1' - there is comedy too, of course. Seeing how Dr. Carter and Maj. Carter interact is something like observing estranged identical twins forced back into the mix again. In addition, the viewer is treated to seeing Teal'c impersonate himself while on an interuniversal mission.