This episode sets up the season finale, and is the first of three linked episodes. Daniel touches a mirror-like device and travels to an alternate reality, one where he is not a member of the SGC and Teal'c, still first prime to Apophis, is sporting a ridiculous ponytail and leading the goa'uld attack on Earth. These alternate-people ultimately let Daniel travel back to the planet with the mirror so he can warn his Earth about the imminent attack, dooming themselves in the process.
This is something that struck me as odd. If an alternate team had come through to our Earth as it was under attack, I doubt we would have given up our hopes of winning so quickly and sent them off to save their friends we'd never met. Granted, alternate-Earth was completely boned by that point, and basically everybody was dead, but they still go for saving Daniel pretty quickly.
It never really made sense to me that the mirror-owning aliens would have known the address for Apophis' base of operations against Earth, either - it's stated at the beginning of the episode that the battle on that planet seems like it was some time ago, and how do they know it was Apophis who destroyed them too? This was an action-packed episode, probably to make up for the following episode, Politics, which is all talk, and so the writers probably weren't too concerned with these plot holes.