This Yar/Guinan episode uses a sci fi plot device somewhat reminiscent of "Mirror, Mirror", but more subtle, in depth, and powerful. The beauty of the episode is that it has such a good hook, it could coast off it for 20 minutes, but instead, the story punches forward with mounting tension, some great character development, and a unique moral dilemma. It's time travel used by the writers for a purpose, with broad universal implications and intimate, personal consequences, as opposed to being just another plot device. And there's a cinematic quality to everything (in some ways foreshadowing the eleventh Star Trek film) that makes "Yesterday's Enterprise" seem bigger than it really is. One of Star Trek's best.