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Captain Kirk and the crew of the Enterprise travel back in time to 20th-century Earth on a special observation mission but soon discover that they are not the only time traveler in town in this second-season episode of the enduringly popular science-fiction series. While in orbit around Earth, the Enterprise unexpectedly comes across Gary Seven, an enigmatic man who recognizes the starship as being from the future. When Gary Seven escapes from the Enterprise and returns to the Earth's surface, Kirk and Mr. Spock decide to go undercover and follow. The two must ascertain the nature of the mysterious stranger's mission before he changes the past forever, while struggling to keep their own identities as visitors from the future secret. Series creator Gene Roddenberry co-wrote the original story behind this episode, which also features an early performance by a young Teri Garr.