Star Trek: The Next Generation: Time's Arrow

Picard and the Enterprise are urgently called back to Earth to see the results of an archaeological dig that reveals aliens may have visited the planet in the 19th century. Among the many relics they have found is one that is totally out of place - Commander Data's head. Data confirms that it is his own head and not that of his brother Lore. He seems completely at ease with the find, which is not the case with the rest of the crew. On the planet, Data encounters a temporal vortex and he suddenly finds himself transported to 19th century San Francisco where he meets none other than Guinan. Captain Picard and crew follow Data back to the late 1800s to get him back, only to find the suspected alien visitors killing people of that time.