"Time’s Arrow" is the 26th episode of the fifth season and the first episode of the sixth season of the American science fiction television series Star Trek: The Next Generation, it comprises the 126th and 127th episodes overall. A two-part episode of Star Trek: TNG, the first episode was a cliffhanger season finale for the fifth season and the second episode was the premiere for the sixth season.
Set in the 24th century, the series follows the adventures of the Starfleet crew of the Federation starship Enterprise-D. In this episode, an engineering team finds evidence that aliens visited Earth in 19th century San Francisco: Data's severed head, buried five hundred years ago.
The second part of the episode was nominated for three Creative Arts Emmy Awards, winning two: Outstanding Individual Achievement in Costume Design for a Series, and Outstanding Individual Achievement in Hairstyling for a Series.
Plot
Part I
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. They are absorbing light strands from a device in the center of the cavern, otherwise appearing benign. When two aliens enter a time portal, Data is drawn in. He finds himself in 1893 San Francisco, California.
Needing money, Data wins a sizable amount in a poker game. He stays at a local hotel, befriending the bellhop (future author Jack London). Claiming to be a French inventor, Data enlists London to acquire 19th century supplies under the pretense of building an internal combustion engine. Data is actually creating a detector to locate the aliens. Data sees a photo of Enterprise bartender Guinan in a newspaper and goes to a reception being held for her. Believing she also traveled from the future, Data approaches Guinan, who is conversing with Samuel Clemens (Mark Twain). Data is surprised when she fails to recognize him, which sparks Clemens' curiosity. While speaking privately, Data realizes Guinan's species is much longer-lived than he realized. She is actually native to this time period and has yet to meet the 24th century Enterprise crew. Data privately explains his situation, unaware Clemens is eavesdropping. Clemens becomes determined to discover the truth about the two.
Meanwhile, in the 24th century, the Enterprise crew build a phase discriminator that allows them to see the aliens and go back in time to rescue Data. Meanwhile, 24th century Guinan convinces that Picard he must join the pending away mission, warning they otherwise may never meet. The away team activates the phase discriminator and see the aliens. The light strands are human life forces at the moment of death that the aliens consume. Picard, Will Riker, Geordi La Forge, Troi, and Dr. Beverly Crusher enter the time portal to try and stop the aliens.
Part II
Arriving in 1893 San Francisco, the away team searches for Data and investigates a cholera outbreak. Dr. Crusher believes the alien shapeshifters use the epidemic to mask their hunting humans to ingest their life-force. The team encounters two shapeshifters at a hospital. When confronted, the aliens escape, which alerts Data to their location and reunites him with the away team. They pursue the aliens to the cavern near San Francisco, followed by Guinan and Clemens. The aliens' cane-like device opens a time portal back to future Devidia II. While struggling over the device, Data's head is detached from his body. The away team follows one alien into the future, bringing Data's headless body and the cane. Clemens spontaneously follows the others to the 24th century, while Picard is left behind, tending an injured Guinan. The dying shapeshifter tells Picard that 19th century Earth would be in jeopardy if the aliens' 24th century world is attacked, due to amplifying the time-shift effect. To send a warning into the future, Picard places iron filings with a binary message into the static memory of Data's head.