The Enterprise's universe turns on its head when an older version of the Enterprise hops into the future through a time rift created by a battle with Romulans. On the new Enterprise, the Federation is at war with the Klingons and Tasha Yar is still alive. Guinan can sense that they are in a "wrong" timeline, and her advice prompts Picard to send the old Enterprise back to its battle and its death, putting the universe back in order.
LAME. And I'm not automatically saying that just because Tasha Yar was in it. I can't say that doesn't prejudice me, but I try to like every episode of Star Trek. I've even tried to like episodes of Enterprise. But that's a different story altogether.
So: lame! I think what I didn't like was Guinan. Not the acting, but the vague things she had to say. "Oh, Captain, things just aren't right!" Pahlease. This is like when Troi used to say that she felt "anger" and then that was all she could tell us. Maybe it would be cool if she could remember exactly what the real time line was like, instead of getting these impressions. Meh. Maybe what I didn't like more is that we still don't know much about Guinan, and in many ways this episode was about her, yet we learn almost nothing about her. Boo.
What I did like was that this episode gives Yar a more meaningful death. It's like the writers are trying to make up for how crappy the writing of her death was in season one. This also has the unfortunate side affect of reminding you of season one.
The best thing about this episode? Christopher McDonald is in it! I spent the entire time wondering where I had seen "Richard Castillo" before, and the answer is every where.
PS. Apparently I have no taste. According to Wikipedia, this episode was one of the higher ranking episodes when it aired and lots of reviewers like it and blah blah blah.