Plot summary
Needing a new ZPM, Colonel Sheppard and his team, joined by Elizabeth Weir, launch a daring mission to the Replicator homeworld to steal a new one while Colonel Samantha Carter and Doctor Bill Lee aid the Apollo in searching for the missing city. While the team is able to steal a ZPM with ease due to Weir's connection to the Replicator network, they realize they can input a command that will force the Replicators to go to war with the Wraith. While they are successful in implementing the new command, Weir is captured, buying them time to escape.
Despite Weir's efforts, the team ends up trapped, surrounded, and without the power to escape to hyperspace. They are unexpectedly rescued by the Apollo which had located Atlantis and came to help. The Apollo lays down covering fire long enough for them to land then carries them back to Atlantis. With the new ZPM in place, the city is piloted to a new homeworld where it is safely landed.
Credits
Adapted from Wikipedia contributors, Stargate Atlantis season 4. HTML formatting and reference markers removed.