Time's Arrow, Part I

Starfleet recalls the Enterprise to Earth after an excavation in a cavern beneath San Francisco uncovers evidence of alien visitors in the 19th century, along with Data's severed head, buried for some 500 years. Cellular traces lead the crew to Devidia II, where they find a temporal disturbance and Troi senses human suffering. Using a phase discriminator, Data sees aliens gathering strands of life energy taken from humans at the moment of death. When two of the aliens enter a time portal, Data is pulled through after them and arrives in San Francisco in 1893. He wins money at poker and, helped by a young bellhop named Jack London, builds a device to detect the aliens. Spotting Guinan in a newspaper photograph, he finds her at a reception talking with Samuel Clemens. He learns that this Guinan has not yet met the Enterprise crew, but Clemens has overheard them and is determined to find out the truth.