Tomorrow Is Yesterday introduces the most classic of Star Trek plot devices: time travel. After the original series, it was also used in the spin-offs (minus prequel show Enterprise) and three of the eleven - so far - Trek movies (The Voyage Home, First Contact and J.J. Abrams' reboot), always to great effect. This episode is no exception, especially as it gets more fun to watch as time goes by.<br /><br />The travel itself is actually just an accident: when a mission goes wrong and hostilities ensue, the Enterprise flies towards the Sun and then away from it as quickly as possible. This, the so-called "slingshot effect", causes the ship to end up orbiting Earth - in the late 1960s! Unfortunately, a pilot working for NASA notices the ship and is taken the hostage by Kirk and Spock, who must now come up with a way to get back home without altering the course of history.<br /><br />The usual elements of every good time travel story are all present: the discovery of a new age, the problems that derive from it and, of course, the discussions regarding possible paradoxes. What really makes the episode stand out, though, is its sense of fun and prescience: ordinary people's reaction to the sight of Kirk and Spock is always a joy to behold, and it's pretty funny to hear our heroes mention man's first landing on the Moon as taking place on a Wednesday at the end of the '60s: they got it right, weekday and all, a full two years before the whole thing happened. Ah, the pleasures of good sci-fi...