A mission from Earth initially led by Dr. Elizabeth Weir, who led a contingent of military and civilian personnel through the Stargate to the lost city of Atlantis, built by the Ancients and resettled in the Pegasus Galaxy several million years ago. The expedition entailed cooperation from more than a dozen countries, and including some of the world's finest minds.
After Daniel Jackson found the gate address to the lost city at the Antarctic outpost of Atlantus, Weir led the expedition through the Stargate. There the expedition reactivated the city and established a long-term outpost there.
After Weir's untimely death the expedition was headed by Lt. Colonel Samantha Carter, and then by the I.O.A.'s Richard Woolsey. Its presence in the Pegasus Galaxy has played a key role not only in the re-awakening of the Wraith, but also in the fight against them on behalf of the galaxy's innocent population.
Rising, Part 1 - Given the go-ahead to travel to the Pegasus Galaxy, the Atlantis expedition commences with the knowledge that they may never be able to return home.
The Return, Part 1 - The expedition comes to a surprising end when a group of Ancients return and reclaim their right to the city.
The Return, Part 2 - After the Ancients are wiped out by the Asurans, Weir and Sheppard lead their team to retake control of Atlantis.
Reunion - Following Dr. Weir's capture by the Asurans, Samantha Carter is assigned to take command of the Atlantis base.
The Seed - With Carter reassigned, the I.O.A. puts one of its own -- bureaucrat Richard Woolsey -- in command of the expedition.