On a mission to establish contact with the reclusive Melcotians, Kirk ignores the message of a space probe which uses telepathy (each crew member hears the message in his own language) to warn the Enterprise to immediately leave Melcot space. Kirk tries to contact the Melcotians, but there is no response to his message.
When Spock, Kirk, Scott, McCoy, and Chekov beam down to Melcot, they materialize in a fog bank not recorded by ship's sensors. In addition, tricorders and communicators cease functioning. The landing party encounters a Melcotian who informs them that they are outside disease which must be destroyed. Their trespassing is to be punished by death, and that the pattern of their death will be taken from Kirk's memories, since it was he who ordered that the Melcotians' warning to be disregarded. Because Kirk's ancestors pioneered the West, the landing party finds itself teleported to Tombstone, Arizona on October 26, 1881. The townsfolk refer to Kirk as Ike Clanton, Chekov as Billy Clayborn, Scotty and Billy Clanton, Bones as Tom McCloury, and Spock as Frank McClary. Spock recalls that at 5:00, on this date in history, the Clanton Gang shot it out with Doc Holiday and Morgan, Virgil, and Wyatt Earp at the OK Corral--and were defeated.
On their way to the local saloon, the landing party witnesses the shooting of a bar patron by Morgan Earp. Inside the bar, Chekov is accosted by a woman name Sylvia who claims to know him. Both Kirk and Chekov narrowly avoid a gunfight with Morgan Earp over the incident. Kirk tries unsuccessfully to make peace with the Earps. When this fails, he and the landing party attempt to leave town so as not to be around at 5:00, but they are stopped by a force field.
Desperately, Bones and Spock cooperate to build a tranquillizer bomb which will incapacitate the Earps. McCoy tries to borrow chemicals for his bomb from the local dentist, only to find that the dentist is Doc Holliday. Nevertheless, Holliday lets him take the chemicals, telling him that he better be finished using them before his 5:00 date with destiny.
Meanwhile, Chekov has fallen in love with Sylvia, with whom Morgan Earp is unfortunately also in love. When Morgan picks a fight over the girl, Chekov is shot and killed. Spock is puzzled by this, since William Clayborn survived the battle at the OK Corral. Kirk takes it to mean that the outcome of the conflict does not necessarily correspond with the historical outcome. Kirk tries to get the Sheriff to call the shootout off, but finds that he is more interested in having the Clanton gang kill the Earps.
With time running out, Scotty volunteers to test the potion Bones has cooked up. Despite Bones' careful preparation, it does not work. Spock is the only one who understands the significance of this fact, saying, ``You do not seem to understand. It did not function. But it must function.'' Spock realizes that nothing around them is real; the whole scenario is taking place in their minds.
Kirk vows not to leave the bar until after 5:00, but finds himself and the others transported to the OK Corral at a few minutes before 5:00. Attempts to leave the Corral reveal that it is encircled by a force field so that escape is impossible. Using a mind meld, Spock convinces everyone else of his certainty that nothing around them, including bullets, is real. So convinced, the Earps shoot their imaginary bullets right through the landing party. When the Earps have stopped firing and after Kirk has spared their lives, the landing party finds itself whisked on board the Enterprise on the way to Melcot with a healthy Mr. Chekov (who is alive because the only thing that was real to him was the girl). The Melcotian probe, once again directly in front of them, emits M waves and then disintegrates. The Melcotians, impressed that Kirk did not kill, then extend an invitation to establish relations with the Federation.