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Acclaimed author Harlan Ellison scripted this time-travel episode of the enduringly popular science fiction series. On the surface of an unknown world, Dr. McCoy -- suffering from an accidental drug overdose -- enters a mysterious archway that announces itself as "the Guardian of Forever." In order to recover their lost crewmate, Captain Kirk and Mr. Spock retrace McCoy's steps, discovering that the gateway is actually a time machine which transports them to 1930s Earth. While there, they encounter anti-war activist Edith Keeler Joan Collins, with whom Kirk falls in love. However, Spock soon realizes that Keeler is destined to die. Kirk struggles with his desire to save her life, knowing that such an action would forever change the course of history, destroying the Enterprise and leaving himself, Spock, and McCoy stranded in the past for the rest of their lives.