Set in the 22nd century, the series follows the adventures of the first Starfleet starship, "Enterprise", registration NX-01. Season three of "Enterprise" features an ongoing story following an attack on Earth by previously unknown aliens called the Xindi. In this episode, the "Enterprise" encounters a version of itself that was sent 117 years into the past—now populated by the descendants of the crew as a generational ship. The two crews must work together so the modern "Enterprise" can access a wormhole defended by Xindi vessels.