“Invasion” is episode nine (originally shown as episode twelve) of season two of Sliders.
At the end of the previous episode, “Obsession”, Derek had detected something about where the Sliders were heading. It doesn’t seem that it was this world, for Rembrandt has picked up some protective equipment between that world and this. They realise the world is a bit too quiet and the word ‘Kromagg’ is written everywhere. Wade wonders what it is; Rembrandt assumes a gang. According to Quinn, something is interfering with the timer and he can’t get a fix on the next window.
Professor Arturo asks if Quinn can locate the source. Yes, and he’ll be able to pinpoint it as they get closer. The place looks like it’s undergone looting, and recently too. Quinn says there’s a powerful EM pulse on the same frequency as the timer as they enter an amusement park. There’s also a noise that Rembrandt and Wade think they should move away from. Quinn says they can’t; the interference is too strong. They need to find the source and stop it, else they won’t know when the next gate is.
Then the professor points out what looks like a spaceship. Quinn says that the timer is pulsing at the same rhythm as the spaceship, which is approaching them. Quinn triggers the timer and the spaceship loses control and crashes.
They head over to the crash and, though Rembrandt and Wade think they should leave, the professor calls Quinn over. Living metal, alien technology from outer space. A man approaches and says not from outer space and asks if they crashed the manta. The Kromaggs will not like that and will avenge its loss with merciless ferocity. The Kromaggs are marauders and killers who eat human eyes. They rode a wave of manta ships and their Earth died screaming. They’re not from outer space; they’re from right here. The man calls to a couple of women, who he calls his twins, and leaves. According to Wade, he was wearing a bracelet from a medical institution on their world. She and Wade still want to leave, but Quinn and the professor enter the ship.
Inside the ship, which has a similar atmosphere to their own, they find a dead Kromagg; humanoid but not human. Quinn feels guilty for killing it. The professor reminds him the Kromagg was an invading soldier. The professor removes a bracelet and an alarm sounds. As alien soldiers arrive, the Sliders flee. Their portal opens and Rembrandt and the professor leave. Before Quinn and Wade do, a red portal opens in the sky and a manta comes out. The Kromaggs are Sliders. And terrible shots.
The next world is an America under French rule. As the Sliders dine, they talk about the Kromaggs. Not alien; every bit as much Earthmen. Different evolutionary paths. The professor mentions a theory of a killer ape, an early primate that killed off all the others. On their world, an evolutionary dead end. On the Kromaggs’ world, not so much. Rembrandt and Wade think people need to be warned. The professor agrees, but how.
On the crashed ship, a Kromagg watches a video of Quinn and the professor inside it.
The bracelet the professor took starts flashing and then a portal opens up and a manta exits. That would make it a lot easier to warn everyone about the Kromaggs. However, the ship is just there for the Sliders and renders them unconscious.
They awaken in a ship and a woman, Mary, explains they are prisoners – then glances at a Kromagg, and corrects herself to guests – of the Kromagg Dynasty. They will be taken to Earth 113 where they will be questioned.
The Kromaggs are powerful, can control Sliding and have serious mental powers as well. Though they lie about their actions, they’re also not very nice and absolutely hate humans. The Kromaggs want to know where the Sliders came from. Presumably to head there and conquer it.