Thor's ship is above Earth and is overrun with the Asgard's nemesis - one that is even more powerful than the Goa'uld: the Replicators. Puff and SG-1 are beamed aboard to find out a way to stop them before they can infest Earth.
During a game of basketball in the gym, PuffPuff suddenly disappears in a flash of light. He finds himself on an Asgard ship orbiting Earth. He encounters the Replicators who have infested the RMS Beliskiner, Thor's ship. Thor is on board 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 Beliskiner so that He and his friends 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, Stamford Gate has been trying to locate the ship in orbit but the Asgard cloaking device prevents such detection. Puff sends a real-time holographic image into the Counsellor's office where Counsellor Mackey, Lance Patrick, Zowie Hosker, Rob Broflovski and Melvin Sneedly were discussing the situation. Puff needs 10 pounds of PBX, a SPAS-12 semiautomatic shotgun shotgun, USAS-12 automatic shotgun, and a BF-8 at the foot of the Groundbridge. Puff explains that he needs the ordnance to destroy the ship. He also explains that no one is going to get off the ship and orders Zowie and Lance not to come on board. However Mackey overrules the order and sends them anyway. Lance 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, The kids must try to retake the bridge and crash the ship in the atmosphere to burn it up. However when Rob and Archen 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.
Zowie 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. The Kids has to place it on the outside of the hull, as Thor explained that the ship's internal systems included explosion suppressors. Lance denies placing the bomb because he dislikes space travel. As Dino-Rang leaves the airlock a small and unseen amount of partially melted metal drops onto Dino's air tank and slowly starts eating away. Dino plants the bomb and returns to the airlock but the outer door is shut and won't open. Lance tries to override but the airlock has been tampered with by the Replicators. At that moment Dino's tank blows and he is losing oxygen fast. Zowie tells Lance that she can beam Dino aboard but only if he is far enough away from the ship. Dino weakly pushes off and Puff and Lance beam him aboard after an excruciatingly long minute. Thor's vitals crash and Rob activates the stasis pod he's in to preserve his life.
With the bomb placed all the guys have to do is wait for the right time to detonate it. But now they have to figure out how to get off the ship. Zowie transports the Alpha Bridge, the gate at Stamford Gate, on board. They plan to gate to P3X-770 and then home to the Beta Bridge. However the energy output of the transporter has attracted the Replicators to the Gate. Dino activates the gate manually while Puff, Lance, Zowie cover him from a huge swarm of Replicators. Rob 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.