The timestream was changed by sending a Doomsday Machine back in time. There,...
Star Trek Phase II
Season 1 Episode 9: Mind-Sifter
A tortured and amnesiac Captain Kirk is dumped into Eath's past by the Klingons, where he ends up in a 1950's mental institution. Meanwhile, a shattered Enterprise crew tries to survive with its new captain: Mister Spock.