“Time’s Arrow” is episode twenty-six of season five of Star Trek: The Next Generation and the season finale.
The Enterprise has returned to Earth on a priority mission, after evidence of extra-terrestrials being there five centuries ago was discovered. Artefacts have been found and Captain Picard and Data are being shown them underground. The pocket watch and eyeglasses are appropriate for the time period, so the captain asks why extra-terrestrials are believed to be involved.
The cavern has been altered by triolic waves, something not found on Earth or used as an energy source by many species, due to a deleterious effect on living tissue. The captain asks why bring the Enterprise home. Because of one other thing they found. Data’s head, severed from its body.
Data is examining the head in engineering; he’s less disturbed than Commander Riker. The captain asks Data if it is his head, and not Lore’s. Data believes so. It’s about 500 years old, around the right time for the other artefacts. Riker tells Data his head is not an artefact. Data thinks it’s clear his life will end in the late 19th century. It has occurred; it will occur.
Geordi hasn’t found out much about whoever used the cave. They’re a species triolic waves will not harm. They might be shapeshifters and it doesn’t look as if they’ve been encountered before. Geordi has found a cellular fossil, though, that comes from only one place; Devidia II. The captain orders a course be set.
Geordi and Data are in Ten Forward. Data has no real need to talk about his head but Geordi does. Data finds it comforting, though. It was possible he might live an unlimited lifespan, which reinforces the fact that he’s artificial. Geordi agrees it must be tough to outlive all his friends. Data expected to make new friends. And outlive them, too. But now he senses completion in his future. He’s not that different to anyone else. He can look forward to death. One step closer to being human.
The captain summons bridge officers and, after Data has left, Guinan comes over. Geordi explains. Guinan says that’s why they went to Earth. She didn’t realise. After Geordi leaves, Guinan makes a comment about full circle. Guinan clearly knows something.
Troi and Riker are heading to the bridge, talking about Data and the effect it has had on the crew. She recites Data’s definition of friendship and is talking about discovering a loved one has a terminal illness when the turbolift arrives, revealing Data. Data has a question. He’s perceiving people behaving differently to him; is this true? It is. They’ll miss him.
There are no signs of live on Devidia II, but a temporal disturbance and triolic waves in its vicinity. The captain tells Riker to lead an away team. One that doesn’t include Data. Data would like to speak to the captain privately. In the captain’s ready room, Data wonders why he isn’t going down, though he knows. Data says you can’t cheap fate. Maybe not, but the captain is certainly going to try.
The away team beam into an empty cavern, though Troi can detect dozens, hundreds of people, all terrified. And human. The ship is contacted and Data reports discovering a synchronic distortion. There’s something out of phase. They could get in phase with a subspace field, but need a phase discriminator. And the only one sensitive enough is in Data’s head.
Data beams down and reports he has altered his communicator so he can remain in contact. He goes out of phase and describes seeing lifeforms around an apparatus releasing energy fragments, which they are ingesting. He also finds an ophidian behind a forcefield. Two entities are approaching and they release the forcefield. Data is detecting a temporal distortion and there’s a noise and the device he used drops into phase, sparking. But no Data.
Data is on a street. He tries finding out where; one person thinks he’s French. Data finds a newspaper, The San Francisco Register. 1893. Data has gone into the past, to the time that his head dated from. He needs to find a way back and the Enterprise is also trying to find him.
The story continues in season six and “Time’s Arrow, Part II”.