“Vengeance” is episode nineteen of season three of Stargate: Atlantis.
Previously, in the season one episode “Thirty Eight Minutes”, Sheppard was bitten by an Iratus Bug, an insect believed to have created the Wraith from humans originally. Dr. Beckett created a retrovirus intended to ‘cure’ the Wraith which was first tested on a Wraith girl in the season two episode “Instinct” and accidentally infected Sheppard in the episode “Conversion”.
The Taranians, who were rescued from a supervolcano in the season two episode “Inferno”, built a new colony on a new world. This colony has now fallen out of contact. Underneath the colony is an extensive tunnel system built by the previous inhabitants.
The team wandering through tunnels with a beeping life signs detector looking for some sort of alien monster is rather reminiscent of Aliens – and the film Alien is in fact referenced. This is not as creepy as it could have been, which seems a wasted opportunity.
It appears that there has been a Wraith experiment ongoing underneath the colony, and that the Wraith/Human infected by the retrovirus, Michael, is also involved.