Time's Arrow, Part II

Picard, Riker, Troi, Crusher and Geordi follow Data through the portal to 1893 San Francisco, posing as a troupe of actors and renting rooms from a suspicious landlady. Crusher suspects the aliens are using a cholera epidemic to hide their killings, and at a hospital the team find two of them in disguise, feeding on the dying. Reunited with Data, they follow the aliens to the cavern, pursued by Guinan and Clemens. In the struggle Guinan is wounded, Data's head is severed, and the others are swept into the future with Data's body, Clemens following them. Picard stays behind to care for Guinan, beginning their long friendship, and hides a message in Data's head. In the 24th century, Geordi reattaches the 500-year-old head, Data reads the message, and the crew find a way to stop the aliens. Clemens, having glimpsed a better future, returns to his own time so Picard can come home, and the Enterprise destroys the alien base.