“Unnatural Selection” is episode twelve of season six of Stargate SG-1.
At the end of the previous episode, “Prometheus”, SG-1 had been sent into the depths of space with the hyperspace drive on the X-303, the Prometheus project, and were lost. Until Thor turned up, and asked for their help, as Replicators had overrun the Asgard homeworld. As the X-303 is of a vastly inferior technology to that of the Asgard, it will not be of any interest to the Replicators. Safety through being primitive.
In “Menace”, SG-1 had discovered the planet on which the Replicators had originated, and the android which created them. The Asgard discovered something in the android’s programming which enabled to summon all of the Replicators to one place. They set a trap, but unfortunately, it failed. They had set a time dilation trap to imprison the Replicators for millennia, but it didn’t work.
The Asgard want SG-1 to go to the homeworld, repair, and activate the device, before the Replicators stop them. SG-1 are the Asgards’ last hope. No pressure.
The device turned out to have really, really not worked, and there’s an enemy that will be familiar to watchers of Stargate Atlantis (not the Wraith) – which series I saw before this, even though it hasn’t started yet in the Stargate timeline. Some things that happened in that series (which I saw before this) now make more sense. An argument for watching programmes in the correct order – if you can.