The Naked Time: Kirk orders Scott to make a full-power restart of the warp engines, a dangerous process that mixes matter and antimatter in a cold state to create a controlled implosion and drive the ship away from the planet. This is suggested by a theory postulating a relationship between time and antimatter, but it has never before been attempted. The restart is successful, propelling the Enterprise at impossible speed away from the planet into a space-time warp that sends the ship back 71 hours in time. While Kirk hopes reliving the last three days is nothing like what they have already experienced, Spock comments that they now know a way to travel back through time. Kirk's response is "We may risk it someday, Mr. Spock."
Tomorrow Is Yesterday: In the episode, the crew of the USS Enterprise travels back to 1960s Earth, however an accident occurs and they must correct damage to the timeline.
The City on the Edge of Forever: In the episode, after a heavily medicated Doctor Leonard McCoy (DeForest Kelley) travels back in time and changes history, Captain Kirk (William Shatner) and Spock (Leonard Nimoy) follow him to correct the timeline. In doing so, Kirk falls in love with Edith Keeler (Joan Collins), but realizes that in order to save his future, he must allow her to die.
Assignment: Earth: Engaged in "historical research", the Enterprise travels back through time to 1968 Earth, where they encounter an interstellar agent planning to intervene in 20th-century events. Kirk and Spock are uncertain of his motives.
All Our Yesterdays: In the episode, Captain Kirk, Spock and Dr. McCoy are trapped in two timeframes of another planet's past.
It guest-stars Mariette Hartley as Zarabeth and Ian Wolfe as Mr. Atoz. The title comes from a soliloquy given by Macbeth.
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That's just TOS. I'm a Trekkie, too, but ST's forays into time travel were a bit much.