Time's Arrow
Summary
The Enterprise is recalled to Earth after evidence is found of aliens on the planet 500 years before. A cavern near Starfleet Headquarters in San Francisco contains 19th century relics and Commander Data's disembodied head. Investigation reveals cellular fossils native to the planet of Devidia II, indicating a race of shapeshifters visited Earth's past. The Enterprise travels to Devidia II, bringing Data's second head. A temporal disturbance is discovered on the planet. Though no life forms are visible, Deanna Troi senses the presence of suffering humans. The crew determine that the aliens are slightly out of phase with time. Data notes that his android body has a phase discriminator that would allow him to see the aliens. Captain Picard reluctantly allows him to join the away team. Inside a cavern, Data, in phase with the aliens, describes them.
Summary from Wikipedia, CC BY-SA 4.0.
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