McCoy is accidentally injected with a stimulant that makes him paranoid and aggressive. He attacks crewmen and beams himself down on a planet the ship is orbiting. There, he jumps into a time travel device that resembles a giant donut and somehow manages to totally disrupt time--so much so that the Enterprise no longer exists. The crewmen who beamed down to look for McCoy before he used the device remain--as if the time change doesn't affect the planet itself but everything else. So it's up to Spock and Kirk to try to use the device to get there just before McCoy so they can prevent him doing something to change time. The problems are timing this exactly right AND figuring out how he impacted history.
Yeah, there seems to be a lot of agreement that this is among the very best (perhaps the best) episodes of the original Star Trek series. And I don't think this is undeserved. Writer Harlen Ellison came up with a dandy script about time travel that is also well-acted and compelling. Guest star Joan Collins is both radiant and a big plus---proving she really could act when she was given a decent script. All in all, a very haunting and moving experience--one not to be missed by anyone--even non-Trekkies.
Yeah, there seems to be a lot of agreement that this is among the very best (perhaps the best) episodes of the original Star Trek series. And I don't think this is undeserved. Writer Harlen Ellison came up with a dandy script about time travel that is also well-acted and compelling. Guest star Joan Collins is both radiant and a big plus---proving she really could act when she was given a decent script. All in all, a very haunting and moving experience--one not to be missed by anyone--even non-Trekkies.