Star Trek: 10 Worst Alternate Realities We've Seen (So Far)

Having all of those extra senses was a blessing and curse for Guinan in Yesterday's Enterprise, aware as she was of the changes in the timeline, though not quite able to pinpoint exactly where and when. A walking ghost, in the form of Tasha Yar, helped her see that nothing was quite as it should be.

The Klingon Empire had engaged in a bloody war with Starfleet for decades, reducing the strength of the fleet by hundreds of ships. In the Prime universe, this had been averted by the heroic actions of the Enterprise-C - sacrificing itself while attempting to save the Klingon colony of Narendra III from Romulans. 

When the ship vanished through a temporal fissure, the Klingons took this as cowardice, thus deepening their hatred for the Federation. As Captain Picard confessed to Captain Garrett in the altered 2366, defeat was inevitable, with surrender seemingly their only option. 

With scant options open to them, the crew of the Enterprise-C voted to return to the past, retaking its place in history, and saving countless lives from a devastating war. If there were one positive thing to come from this horror, it was the heroic and meaningful death of Lt. Tasha Yar - eventually.