SG-1 founds one interesting artifact on totally dead planet where is no oxygen. The decide to take it with them and investigate it. But things get bad when the artifact is heating up. They try to get it back trough the Stargate but the thing stops thats and hurts O'Neil. Soon, they all find themselves infected by some kind of virus.
What comes later - the intense search for the answer, the way Carter first tells not to do the things she later asks and the way they got the whole thing wrong way and the way the solved it. For some time it did looked hopeless. Great episode.