S4, E12: High above the Replicator homeworld a provisional alliance wages an all-out assault on the Replicator fleet. As the battle wages a group of wraith execute a last-minute plot to retrieve a ZPM from the planet's surface. Weeks later the Atlantis long-range sensors detect the signal of a subspace tracking device familiar to Dr. McKay. The team follows to signal to a damaged hive ship drifting in space. Amidst the ship's records they discover references to a secret wraith base...