When Commander Spock suspects that the Klingons are "up to something," he engages in a bit of espionage to find out what they are planning. He manages to send two very cryptic transmissions back to the Enterprise before he is captured, and the Klingon ship travels through a strange phenomenon in that region of space, and vanishes.
Spock is found unconscious by Aaron Stemple, a wealthy but hard-edged miller living in Seattle, Washington. The year is 1865. All Stemple knows is that this man is a stranger, an alien, and injured. Stemple cares for the injured man, and when Spock awakens with no remembrance of who he is, only that he believes he has something that he has to do, he takes him in as his nephew, calling him Ishmael Marx. Living as Ishmael, Spock wrestles to find a new life in place of the one he has lost. He befriends several women who have been brought to Seattle to wive the settlers, attempts to assist three brothers buy their way out of a bet with his "uncle" over the women becoming married by the end of the year, he works as the accountant at the mill - all the while he is plagued by a vague sense that there is something he must do, yet he does not know what it is. When Klingons arrive uninvited at a wedding he is attending, everything comes back to him, and he must take immediate action to prevent all of history from being corrupted.
Meanwhile, following the clues that Spock had transmitted, Captain Kirk and the Enterprise crew race to solve the riddle of where and when the Klingons have gone. Aided by a Vulcan historian who had worked closely with the Klingon historian they got their information from, they determine that the aim of the Klingons was to eliminate the person who had prevented a takeover of Earth by hostile forces in the 1870's. They then set about attempting to recreate the time travel which the Klingons had done, in order to prevent them from killing one Aaron Stemple.
This report prepared by Tabitha Higgins