"Demon With a Glass Hand" is an episode of The Outer Limits that first aired in 1964. Significantly, it was written by "fantasist" author Harlan Ellison (who hated being called a sci-fi author). Harlan Ellison has many high-profile and famous projects under his belt, including another Outer Limits episode, "Soldier", and worked as a consultant on Babylon 5. The cast consisted mostly of actors I'd never heard of, although viewers of a certain age will recognize Robert Culp as Trent, who co-starred in the classic television series I Spy.
Widely considered to be one of the best episodes of The Outer Limits ever filmed, "Demon With a Glass Hand" is unfortunately exposition-heavy, trying to cram an enormous amount of backstory into an hour-long show. This episode could easily have been padded out into a miniseries. The action comes to a halt at various points so the titular character, Trent, can get filled in as to what's going on and what he's supposed to do.
The standard opening narration gives us context. The man who never dies is a recurring theme in mythology doomed to wonder alone forever. We'll soon see that our main character, Trent, is an immortal of sorts. The audience hears his internal monologue via voice-over, informing us that he's being chased by strangers and doesn't know why, having no memory beyond ten days ago. But Trent has one advantage, his two-fingered left hand, made of glass and containing blinking lights, advises him on the best course of action in a robotic voice. It advises him here to remove the medallions from his pursuers, and find its three missing fingers.
Getting the jump on one of them, he yanks off the medallion, and the being vanishes into thin air. Soon he's able to capture and interrogate the other.
What follows is the first of several breaks for a heap of exposition to be unloaded into the story. The aliens and Trent are from 1,000 years in the future. The aliens have conquered Earth in just 19 days, and Trent escaped into the past. All of the aliens have gold medallions which anchor them in the past, removing the medallions will return them to the future. And Trent's glass hand holds information which is important to everyone. His only chance to survive the next few days is to destroy the time mirror, the device which allowed the aliens (only two at a time) to follow him into the past. And he must get the three missing fingers back from them.
While running from more aliens, Trent runs across a woman named Consuelo who will become his sidekick for the remainder of the episode. As he continues to defend himself against the aliens, his backstory continues to fill in a bit at a time, giving him a clearer picture of who he is and what he's doing here. Soon he's cornered in an old office building with no obvious way out, and his glass hand gives him an odd bit of advice, "Let them kill you."
Stepping out into the open, he allows one of the aliens to shoot him, and collapses to the ground, dead. The alien then re-attaches one of his missing fingers, intending to return to collect the hand later. As this was all part of the hand's plan, it talks Consuelo through a procedure to resurrect Trent. It gives a running report of Trent's vital signs during the procedure, which sound entirely too high to be normal, but he eventually regains consciousness.
With one of the missing fingers replaced, the hand can unload another clump of exposition. The future population of the Earth was 70 billion people, all of whom vanished after the aliens invaded. When complete, the hand can reveal what happened to them. Trent needs to protect this information from the aliens, who want to use it to complete their conquest. Now that the aliens think Trent is dead, they will certainly bring the remaining two fingers back to the present with them, from the future where they have been keeping them hidden, to complete Trent's hand.
Soon Consuela manages to rip the medallion from another alien, and Trent obtains the second of the missing fingers. With the second finger, the hand reveals that mankind's last desperate attempt to drive off the aliens was to release a deadly radioactive plague. Unfortunately, as the plague is an indiscriminate killer, all 70 billion humans needed to be digitized into data and stored on a gold-alloy wire to keep them safe for 200 years until the plague dies out. The wire was entrusted to Trent for protection, but he doesn't know where it is. The hand needs its final missing finger in order to reveal this last, critical bit of information.
In the middle of all this adventure, Conseuelo soon falls in love with Trent. Trent, unfortunately, doesn't feel the same way toward her. He hides her on a ledge outside the building to keep her safe, and goes off to face the aliens alone. The aliens, it is revealed, are dying of the plague, and they need Trent's hand. Just as the leader of the aliens comes back to the present with the last finger, Trent manages to destroy the time mirror, ensuring no more of the aliens can come through. The alien leader tries to hide the finger from Trent, but Trent finds it and finally completes his glass hand.
After Trent and Consuelo defeat the remaining aliens, they ask the hand for the last clump of exposition. The gold-alloy wire, containing the entire human race in digitized form, is inside of Trent. Throughout the episode, Trent has exhibited superhuman competence and unusual vital signs, was able to be resurrected from seeming death by gunshot, and found himself unable to love Conseulo. The hand explains all of this: Trent is a robot, designed to be the guardian of the human race. The truth revealed, Trent finds himself doomed to 1,200 years of loneliness to await his destiny in the distant future, to survive the alien invasion, wait out the radioactive plague, and ultimately restore the human race.