Stargate Atlantis, sometimes called SGA, was the first spinoff series from Stargate SG-1, set in the Pegasus Galaxy on the lost city of Atlantis, which was built by the race known as the Ancients, before they Ascended to a higher plane of existence. Evidence of the Ancients had been encountered before – for one thing, they were the race who had built the entire stargate network – and their technology was responsible for the final defeat of the Ori, as well as being what made Anubis so dangerous.
Getting to Atlantis was difficult, and the Atlantis team suffered from a lack of resources, as well as lacking the ability to communicate with the SGC, at least initially, before contact could be re-established with the Milky Way. Lack of a viable hyperdrive meant that the stargate had to be used, and there simply wasn’t enough power to open it up again. The first episode of the series is set between “New Order: Part 2” and “Lockdown”, episodes two and three of season 8 of Stargate SG-1, and a number of events are referenced in each series, so SG-1 and Atlantis should properly be watched concurrently during the period that they overlap, seasons 8-10 of SG-1. If they aren’t, events do not make as much sense as they should.
As in SG-1, there were few genuinely alien races in Atlantis; there was one major foe for the series, known as the Wraith, vampiric beings who consume the life force of humans, ageing the human – until death if they fed for long enough – and prolonging their own lives in return.
With it being set in the same universe as SG-1, there was some crossover of characters between the series. Dr. Elizabeth Weir, who was first introduced in the Stargate SG-1 episode, “Lost City: Part 1”, and briefly ran the SGC, had a major role in this series, but the character was played by a different actress to the one who appeared in that episode. Dr. Rodney McKay, who had a number of recurring minor appearances in SG-1, where he mostly annoyed Sam Carter, is one of the major characters in this series. Richard Woolsey (played by Robert Picardo, known for playing The Doctor in Star Trek: Voyager), who also appeared in SG-1, had some regular appearances in Atlantis and also had a larger role later on. Dr. Daniel Jackson, from SG-1, appeared a few times and some of the Atlantis characters also appeared in the SG-1 episode “The Pegasus Project”. Following the series, several actors also had roles in Sanctuary, and a regular guest star is Mitch Pileggi, who played Assistant Director Walter Skinner in The X-Files.
The Ancients in this series are portrayed as being not entirely competent (they are sometimes met in person as well, unlike in SG-1 where they were all Ascended); though they had advanced technology, they also had a tendency to make errors, sometimes really major ones, resulting in problematic outcomes when some of their technology was used and the after-effects of other things that they had done.
Stargate Atlantis continued after the cancellation of SG-1 and ran for five seasons, after which SGU Stargate Universe commenced. After SGA was cancelled, there was intended to be at least one direct-to-DVD film, called Stargate: Extinction, for which the script was written, with more planned if it was successful. However, following the cancellation of SGU Stargate Universe, Extinction was also cancelled. Fortunately, Atlantis did not end on a cliff-hanger, unlike SGU, or with the dangling plot threads like at the end of SG-1.