Nemesis

Plot
The episode starts out with Jackson resting in the medical room because his appendix has been removed. Thus the rest of SG-1 gets leave time. Teal'c is offworld visiting his son and Col. O'Neill tries to invite Major Carter to a fishing trip in Minnesota. Carter follows O'Neill halfway through the base to wish him a good time when he disappears in a flash of light. O'Neill finds himself on an Asgard ship orbiting Earth. He encounters the Replicators who have infested the Beliskner, Thor's ship. Thor is onboard and explains that the Replicators took over his ship. The navicomputer contained the coordinates for Earth and they set a course for it. Thor is also dying but has made recordings about the technical specifications of the Beliskner so that O'Neill might find a way to stop the Replicators from landing on Earth. Thor also disabled the outbound transporter beams to prevent the Replicators from escaping.

Back on Earth Stargate Command has been trying to locate the ship in orbit but the Asgard cloaking device prevents such detection. O'Neill sends a realtime holographic image into the SGC conference room where General Hammond, Carter and Teal'c were discussing the situation. O'Neill needs 10 pounds of PBX, a SPAS-12 shotgun, USAS-12 shotgun, and a BF-8 at the foot of the Stargate. O'Neill explains that he needs the ordinance to destroy the ship. He also explains that no one is going to get off the ship and orders Carter and Teal'c not to come on board. However Hammond overrules the order and sends them anyway. Carter explains that a shuttle is ready to launch and can reach them in a matter of hours as long as they have the correct coordinates.

After more discussion with Thor, SG-1 must try to retake the bridge and crash the ship in the atmosphere to burn it up. However when Teal'c and O'Neill reach the bridge it is completely overrun with Replicators and there is no chance of even reaching the navicomputer let alone putting in a new course. So they must make a new plan.

Carter and Thor then suggest putting an elementary naquadah enhanced bomb (BF-8) on the deceleration drive and detonate it when the ship enters the atmosphere. SG-1 has to place it on the outside of the hull, as Thor explained that the ship's internal systems included explosion suppressors. Teal'c volunteers to place the bomb because his Goa'uld symbiote will protect him from possible radiation around the ship. As Teal'c leaves the airlock a small and unseen amount of partially melted metal drops onto Teal'c's airtank and slowly starts eating away. Teal'c plants the bomb and returns to the airlock but the outer door is shut and won't open. Carter tries to override but the airlock has been tampered with by the Replicators. At that moment Teal'c's tank blows and he is losing oxygen fast. Carter tells O'Neill that she can beam Teal'c aboard but only if he is far enough away from the ship. Teal'c weakly pushes off and Carter beams him aboard after an excruciatingly long minute. Thor's vitals crash and Carter activates the stasis pod he's in to preserve his life.

With the bomb placed all SG-1 has to do is wait for the right time to detonate it. But now they have to figure out how to get off the ship. SG-1 transports the Alpha gate, the gate at Stargate Command, on board. They plan to gate to P3X-234 and then home to the Beta gate. However the energy output of the transporter has attracted the Replicators to the Gate. Teal'c activates the gate manually while O'Neill and Carter cover him from a huge swarm of Replicators. O'Neill blows the bomb and the ship tumbles out of control and crashes in the Pacific Ocean. However while the ship sinks we see that one Replicator survived.