“Lifeline” is episode two of season four of Stargate: Atlantis.
After the events of the previous episode, “Adrift”, much of the city of Atlantis is outside the shield. The Atlanteans take Dr. Weir to the human-form Replicator homeworld because of the Replicator nannites in her which will help them steal a ZPM. Colonel Sheppard leaves Teyla behind in charge of the city, taking McKay and Ronon as well.
Meanwhile, Colonel Carter has met up with the Apollo in the Pegasus galaxy, and reconfigured the ship’s sensors to help find Atlantis.
Unfortunately, by the end of the episode it looks like another major character has been lost, either killed off or relegated to a minor role. Which, isn’t known yet. That’s the third one that’s gone if that’s the case, and the second in only a few episodes, as Dr. Beckett was killed off in “Sunday”.