At some point, this Goa'uld, who was an agent for the Trust, managed to take Colonel Steven Caldwell, the commander of the Daedalus as a host to infiltrate Atlantis. The Goa'uld managed to copy the Atlantis command program and then delete the record of doing so from the Atlantis log, successfully avoiding being detected in the process. The Goa'uld then returned to Earth where the Trust's Goa'uld scientists altered the program to overload the Atlantis ZPM and destroy the city to keep the Wraith from reaching the Milky Way Galaxy. Returning to Atlantis, the Goa'uld once again accessed the Atlantis mainframe undetected and uploaded the altered program, putting in an access code that would allow only it to disable the program and once more deleting the record of its actions.
When the Goa'uld attempt to destroy Atlantis ultimately went down, the Goa'uld was returning to Earth on the Daedalus. However, Agent Malcolm Barrett and the NID learned of the Goa'uld plot and alerted Stargate Command to the threat. Stargate Command then contacted the Daedalus to act as a relay to warn Atlantis not to dial Earth. Though the Goa'uld's mission was to destroy Atlantis, it commanded the Daedalus to undertake the mission to warn Atlantis, presumably to avoid drawing suspicion upon itself. When the Daedalus is recalled to Atlantis, the Goa'uld pushes the idea of Doctor Peter Kavanagh being the Trust agent which is believed to be a human being, not a Goa'uld. The Goa'uld's program goes on to send out a distress signal to draw in two nearby Wraith cruisers in order to force the expedition to reconnect the ZPM to power the cloak at which time the program being powering up the Stardrive's inertial dampeners which will overload the ZPM and destroy Atlantis as planned.
As the Goa'uld prepares to take most of the expedition members to the Light Bugs' Planet for safety on the Daedalus, Lieutenant Laura Cadman finds the deletion points with Caldwell's identification code and identifies him as the saboteur. At Doctor Elizabeth Weir's request, Hermiod beams down "Caldwell" where he is confronted by Weir, Lt. Colonel John Sheppard and Ronon Dex about his actions. The Goa'uld reveals itself and proclaims that it will never give up the access code. In response, Ronon knocks the Goa'uld to the ground and Sheppard hits it with a taser. The electric shock enables Caldwell to take back control from the Goa'uld and Sheppard asks for the access code from Caldwell. Though visibly struggling for control, Caldwell is able to give Sheppard the access code the Goa'uld used and the attempt to destroy Atlantis is foiled just in time.
Following the foiling of the plot, Hermiod uses the Daedalus Asgard transporter to remove the Goa'uld from Caldwell. Despite the incredibly intricate calculations necessary, Hermiod is able to safely perform the extraction and the Goa'uld is presumably killed afterwards. (SGA: "Critical Mass")
Some time afterwards, Caldwell returned to Atlantis with full memory of what the Goa'uld did while controlling him and wanting to make amends. While Weir was willing to forgive Caldwell as she knew what he did wasn't his fault but the Goa'uld's, everyone else was more hesitant to trust him with Phoebus and Thalan using the Goa'uld as an example of why Caldwell couldn't be trusted while possessing Weir and Sheppard. Later, Caldwell tells the two that he is the one person who can understand what they went through because of the Goa'uld's possession of him. (SGA: "The Long Goodbye")
- As it is unknown how long the Goa'uld controlled Caldwell, it is possible that the Goa'uld was responsible for some if not all of Caldwell's early antagonism towards the Atlantis expedition. The Goa'uld controlled Caldwell for at least a couple of months and his friendlier attitude after the Goa'uld was removed supports the theory that the Goa'uld was the true reason for Caldwell's early behavior.