The discovery of Data's head, old, dusty and dead, among the ruins of some 19th-century artifacts in San Francisco leads the crew on a wild-goose chase through time. In the 24th century, they have found a race on the planet Devidia II who can travel through time and assume human form on Earth. Meanwhile, Data has become trapped in 19th century San Francisco with a 19th century Guinan and a very curious Samuel Clemens, prompting Picard, Riker, La Forge, Crusher and Troi to follow, in an attempt to rescue their friend.

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