“End of Eternity” is episode twelve of season one of Space: 1999.
An Eagle has landed on an asteroid and charges are being placed. Victor tells the commander that there is atmosphere inside. Paul contacts them from Main Mission; the nearest system is three lightyears away and if the asteroid came from there, it would have taken at least a thousand years to get where it is now. The charges are in position and the commander orders them triggered.
This reveals a door, which is opened. The chamber beyond is an airlock and pressurises with breathable air. Koenig tries shooting the next door open, but nothing happens. Until he walks away to get some explosives, at which point the door explodes by itself, knocking one of the crew, Mike Baxter, down.
The chamber revealed has paintings, rather disturbing ones, and someone pinned under debris. The person is bandaged and placed in a pressurised container to be taken back to Alpha, though Alan doesn’t think he’ll make it. Victor takes a rock sample and comments that there’s nothing else there; a one-room world. Back on Alpha, the alien is taken to medical, but dies.
Koenig is talking to Baxter; Baxter needs checking out by medical until he’s cleared to fly again. Dr Russell starts doing an autopsy on the alien, but removing the bloodstained bandages from his head reveals no damage. She contacts Koenig and, when he arrives, tells him that the alien has healed. He was dead but is now alive. Dr Mathias has some results; the doctor explains that the alien has no cell decay. His body is in some kind of stasis. Koenig asks if he could have survived for a thousand years. Yes, it possible. The commander is going to post security.
Victor is showing the commander the rock sample. He disintegrated it with antimatter, but it keeps reforming once the antimatter bombardment stops (really, it should have caused a massive explosion). It regenerates. Like the alien. Who has now woken up. Victor thinks the rock is a living organism, and the commander wonders if it is organic or created. Victor suggests it was placed to protect the alien, to prevent people from getting it. Or the alien from getting out, according to Koenig. The alien has now left medical and taken down the two guards posted.
Dr Russell is checking Baxter out. She needs to wait for the results. Commander Koenig goes to see him once they’re finished; Baxter is permanently grounded from flying because his optic nerve was seriously damaged in the explosion.
Dr Mathias finds the two reviving guards and that the alien is gone. He contacts the commander. When the alien’s location is determined, security is dispatched. They fare no better. Alan and Koenig arrive and Alan grabs a guard’s gun. This has no effect on either the stun or kill settings. The alien finally talks and asks them to stop. He must talk to them.
The alien explains his name is Balor. His people managed to defeat death and achieve immortality. But this caused a decline in their civilisation, and Balor tried to make them aware of this. He was exiled for his troubles. Victor and Dr Russell agree the story fits the facts, but Koenig thinks there’s more to it. Because of the unpleasant paintings, if nothing else. And he’s right. Balor’s methods for getting people to understand death were not appreciated amongst people who could not die. They are even worse for people who can be killed.