Time's Arrow (Star Trek: The Next Generation)

The Enterprise is recalled to Earth on a priority mission pertaining to evidence of aliens on the planet 500 years before. They are shown a cavern near Starfleet Headquarters in San Francisco containing relics of the 19th century, and the severed head of Data. The Enterprise crew finds evidence pointing to a race of shapeshifters and cellular fossils native to the planet of Devidia II. Taking Data's second head, the Enterprise travels to the planet, and discovers a temporal disturbance on the planet. Though no life forms are visible, Deanna Troi senses the presence of suffering humans, and the crew work out 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, and Captain Picard reluctantly allows him to do so. Once in phase with the aliens, Data describes them to the crew as absorbing strands of light from a device in the center of the cavern but otherwise appear benign. However, as he observes, two aliens enter a time portal that he is drawn into; Data finds himself on Earth in San Francisco in the late 19th century, August 11, 1893.

Data quickly surmises that he needs money to operate, and is able to win a sizable amount beating card sharks at their own game in poker. Taking residence in a local hotel, Data claims to be a French inventor, befriending the bellhop (future author Jack London) for help in acquiring parts to build a detector to find the aliens, using 19th century technology. Data finds Guinan, the bartender from the Enterprise in a newspaper photo, and goes to a reception she will be attending. As she is speaking with Samuel Clemens (Mark Twain), Data attempts to ask her questions about the future believing her to have followed him back in time, but discovers that she is native to 1893 and has yet to meet the Enterprise crew; Data's language intrigues Clemens and he begins to follow Data and Guinan around.

Meanwhile, the Enterprise crew has determined how to build a similar phase discriminator to allow them to also see the aliens and go back in time to rescue Data. Guinan urges Picard to go along with the away team as, otherwise, they will have never met in the past and could change history. Picard and the rest of the away team activate the phase discriminator and see the same aliens as Data described, but discover that the strands of light they are consuming are human life forces, taken at the moment of death. The away team uses the time portal to travel back to the past to put a stop to the aliens.

Arriving in 1893, the away team quickly locates Data, who explains the current situation. Using Data's device, the team, followed by Guinan and Samuel Clemens are able to follow the alien shapeshifters to the same cavern near San Francisco, where they discover that the aliens have traveled to the 19th century to take advantage of a cholera outbreak, draining the life force from humans in infected areas to give the impression that their deaths were the result of an epidemic. In an ensuing struggle over a cane-like device used to open the portal, Data's head is detached from his body and left in 1893, and Guinan is injured. As Picard tends to Guinan, the rest of the away team, carrying Data's body which continues to grasp the cane device, follow one of the aliens to the future, with Clemens also following them. Picard learns from the other alien that should the Enterprise destroy their base in the 24th century, the strength of the time shift will be amplified, potentially devastating 19th-century Earth. Picard places a binary message using iron filings in Data's static memory to leave instructions for his crew in the future.

In the 24th century, Geordi La Forge reattaches Data's 500-year-old head onto his body, and Data discovers Picard's message. Data and LaForge devise a way of putting photon torpedoes in phase with the alien habitat, and therefore avoiding the amplification of the time shift effect, while Riker decides to attempt to rescue Picard. As studies of the cane device reveal that, due to instability of the portal, only one person would be able to return from the 19th century, Clemens offers to go back to save Picard. Clemens returns to the cavern in the 19th century, giving Picard the device to operate the time portal himself, and offering to tend to Guinan's wounds. Picard thanks Clemens, and explains that he wishes he could have gotten to know him better, but Clemens points out that his personality is written into his books. Picard returns to the future just as Riker orders Worf to fire the reconfigured torpedoes at the habitat, and is beamed out just in time.

Writing for The Deseret News, television editor Scott D. Pierce found the first part of the story "fresh and intriguing".[3] Wired asked readers to select which episodes of the series were their favorites, and "Time's Arrow" was highlighted in their resultant article.[4]

Variety listed "Time's Arrow" (Parts I & II) as one of the top 15 episodes of Star Trek: The Next Generation.[5]

In 2016, the "Time's Arrow" two-part episode was ranked by SyFy Wire as the 9th best Star Trek franchise episode involving time travel.[6] Mark Twain (Samuel Clemens) was ranked as one of the top seven time travelers of the franchise by Nerdist.com in 2019.[7] They note that when he is taken to the future, he is glad there is no poverty, war, or prejudices.[7]

In 2016, Empire ranked this the 32nd best out of the top 50 episodes of all the 700 plus Star Trek television episodes.[8]

In 2018, CBR ranked this one of the top-twenty time travel themed episodes of all Star Trek series.[9]

In 2020, SyFy Wire noted this episode the relationship between Picard and Data, in particular showing that Picard is reluctant to risk what they call his "robot bestie".[10] They point out the episode begins with the shocking discovery of Data's head on Earth, which causes a certain concern among the crew that Data will die on ancient Earth somehow, leaving this head to be discovered in the 24 century.[10]

The second part of the episode was nominated for three Creative Arts Emmy Awards, winning two:

In his book Time Travel (2012), author David Wittenberg wrote favorably of the depiction of the logic of time travel in the episode: "Star Trek's 'Time's Arrow' is both cognizant and respectful of … physical theory, offering a time travel loop in which causal order is not upset, or, in other words, in which no strictly logical paradoxes ensue."[12]