Time's Arrow Part. I & II (Star Trek: The Next Generation, Season 5, Episode 26 and Season 6, Episode 1)
There's a lot to say about this two-part episode, which won two Creative Arts Emmys. Time travel was a recurring theme in all the series. In this case, Data went back to 1893, where he met Mark Twain, Jack London, and a younger version of Whoopi Goldberg's character, Guinan. They discover aliens are taking advantage of a cholera outbreak to steal life forces and dilate time. An Enterprise away team goes back in time to save Data using his severed head -- right, we should have mentioned, they found Data's severed head in a cave at the beginning of the episode -- to communicate 500 years back and forth in time.
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