Rather than there being an enlightened Federation, Earth rules its corner of the universe as the barbaric Terran Empire. They encounter brutality, lust, and the drive for power, and you can feel the tension as they try to get to their own universe before the rest of the "evil" crew discovers the truth. The only thing that remains constant is a logical Spock, showing that shows logic stays the same whether good or evil.

Although, Spock isn't identical, for as we all know evil wears a goatee!

With a notable nod of almost inclusion towards This Side of Paradise, and the two-part story The Menagerie, Ito begin my top 5 countdown with another Season 1 episode. The story which first introduced us to Khan Noonien Singh (played by Ricardo Montalban) is also an important one for laying out many important aspects of the history of the Star Trek universe.After the crew of the Enterprise encounter a 200 year old, pre-warp drive earth vessel derelict in space, they find dozens of people in suspended animation. As the ship begins to power up and the stasis compartments begin to fail, Dr. McCoy saves one man from the brink of death. Khan, a former world leader, has been genetically engineered to be smarter, stronger, and more aggressive than normal humans.The story from then on out is a little predictable, until the excellent finale, but it's one of the most satisfying Star Trek episodes for both hardcore fans and casual viewers alike. Mainly thanks to a brilliant script and excellent performances from William Shatner, Leonard Nimoy, and of course a perfectly weighted display of intensity from Ricardo Montalban.What we have here is 45 minutes of perfectly executed suspense, with intelligent real-world parallels (Cold War), spot-on characterisations and the introduction of Star Trek's second most important hostile alien race after the Klingons: the Romulans. And, by the way, I'd like to argue the fact that no other recurring hostile alien race had such a brilliant introduction episode as was given the Romulans here.After receiving a distress call from a Federation outpost, the Enterprise is dangerously close to the Neutral Zone which, if crossed, would lead to open conflict with the Romulans, although at this point in time no one has ever actually seen them in the flesh. Soon enough, a Romulan vessel appears, carrying a new weapon and a cloaking device which makes it nearly impossible to defeat. Facing the threat of imminent annihilation, Kirk must engage in a battle of wits with the Romulan Commander (played by Mark Lenard) to ensure the survival of his crew. Unfortunately, the task is made more difficult when one of the men accuses Spock of being in league with the enemy, due to the physical resemblance between Romulans and Vulcans, two races that are, in fact, distantly related (a fact that is quite ironic with hindsight, given Lenard went on to play Spock's father Sarek starting with Season 2).The mirror universe idea is not unique, or indeed anything new, but it's maybe utilised here better than in any other on screen attempt.After a storm causes a transporter malfunction, Kirk, Uhura, Scotty, and McCoy are sent to an Enterprise in a parallel, but opposite, universe.A firm favourite of mine, from childhood to today. The story, although a little silly on the surface (in essence, Kirk fights against a man inside a poor reptile costume), is actually a wonderfully crafted insightful critique on the unnecessary casualties of war.After a Federation outpost is destroyed, Kirk and crew go after the alien attackers. During the pursuit, the Enterprise is seized by more powerful beings who send Kirk to a desert planet so that he can face his adversary: the reptilian Gorn. Whoever wins the duel will be set free; the loser will be destroyed, along with his ship and crew. Given the Gorn is much stronger than Kirk, the latter has to come up with an inventive strategy to survive.Kirk’s final act is both heartbreaking and beautiful.All the previous four episodes, and the two I mentioned in opening, may jostle for position amongst places 2 thru 5, and on any given day there's a good chance that The Conscience of the King, Amok Time, All our Yesterdays, and Who Mourns for Adonais? might trouble the lower numbers. But this one episode would always absolutely come out on top.The City on the Edge of ForeverStar Trek’s greatest single episode in any series (TOS, TNG, VOY etc). It single handedly threw down the challenge to future storytellers to not use time travel as a mere gimmick but as a prismatic tool for examining history and why we made certain choices along the way.When an accident causes Dr. McCoy to go temporarily insane, he escapes to a strange planet. There, the search party discover a device left by a superior, vanished civilisation. It's a time portal which plays the history of Earth for them. Drugged and psychotic, Bones jumps through it into the past, causing a change in history important enough to make the Enterprise vanish.Kirk and Spock then travel back in time to 1930s Earth looking for McCoy. They meet a charity worker named Edith Keeler (played by Joan Collins) with whom Kirk (and by now we know what kind of ladies man the Captain is) falls in love with. It's the middle of the Great Depression and Keeler dreams of a future which Kirk knows will one day come true. But her destiny is to die in a car crash within days, and if Kirk stops her death she will go on to lead a pacifist movement that will prevent the United States from entering World War II, and thus allowing the Nazis to conquer the world.The last act is incredibly hard to watch, time travel has never been so emotional! Simply, Kirk must face the truth that Edith has to die, so that the world he lives in, and that she dreams of, can exist.