Star Trek was one of the first TV shows to dabble in timey-wimey goodness, and it's a testament to the show's writers and actors that this classic episode remains completely captivating almost 50 years after it originally aired. When a delirious McCoy accidentally stumbles through a time portal and alters human history to such an extent that the Enterprise no longer exists, Spock and Kirk are forced to go after him and undo the damage he's done. On the other side they find themselves in 1930s America. They soon set about tracking down McCoy, but Kirk also clears enough time in his schedule to fall in love with a lady. If we've learned anything so far, it's that time travel and love go together as well as Batman and Father's day. Of course, it all ends badly. Kirk's new girlfriend was meant to die in a car accident, but didn't. This sets off a chain reaction of events that lead to the Nazi's winning the Second World War and everything being horrible for everyone. In a truly heartbreaking scene, Kirk stops both himself and McCoy from rushing out to save her when she crosses the road to meet them. Kirk can only watch helplessly as the only woman he's ever loved gets knocked down by a car and dies right in front him. Ouch.
