“Nemesis” is episode twenty-two of season three of Stargate SG-1 and the season finale.
Daniel is laid up in the hospital after having his appendix removed and everyone else is on vacation. O’Neill is getting ready to head off on a fishing trip when Thor beams him into an Asgard starship, only to find it infested with small, insectile (albeit rather big insects) robots.
Thor is rather poorly – in fact, he is dying. The robots are the Replicators, the enemy of the Asgard that is even worse than the Goa’uld that had been mentioned previously. They are the reason why most of the Asgard fleet is unavailable.
The Replicators intend to land the ship on Earth, which would be very bad. O’Neill has to destroy the ship before the Replicators manage to accomplish this. Preferably without killing himself at the same time. The Replicators are aptly named; they consume things and make more of their kind from what has been consumed.
This is the first appearance of the Replicators. The story doesn’t conclude in this episode, but continues in the next episode, “Small Victories”.