The Doctor and Sarah visit the planet Zeta Minor on the edge of the universe.
On a dark and distant planet, a spaceman named Braun steps out of a small base unit and plants a sign in the jungle. It carries the name and date of death of Egard Lumb. Unearthly howls echo around him and he points a rifle warily at his surroundings before returning to the base. Meanwhile, elsewhere on the planet, another spaceman, Baldwin, finds crystals near a pit. He hands them over to Professor Sorenson.
Baldwin receives a call from Braun; it is nearly night, and they need to get back to base quickly. Sorenson notes the vein they have uncovered is almost seventy percent pure. The last time they hit a vein this rich, Lorenzo died and the vein vanished. Sorenson says the planet is alive and took the vein back, but vows he will not be beaten again. Unable to persuade Sorenson, Baldwin leaves him. At base camp, Braun is attacked by an invisible force. He vanishes, screaming. When Baldwin makes it back to the base, he is also attacked, just managing to send a distress signal before he, too, vanishes.
In the TARDIS, Sarah wants to know what is wrong. The Doctor had promised they would be back in London five minutes before they left Loch Ness. The Doctor admits that they have emerged from the time vortex thirty thousand years too late. The TARDIS receives Baldwin’s distress call and the Doctor lands the ship. They emerge in the jungle, the Doctor tracking the signal with a small device. Suddenly, Sarah seems transfixed by a strange noise and stares straight ahead at something unseen until the feeling passes.
A probe ship approaches Zeta Minor, the last planet in the known universe. The ship’s controller, Salamar, assigns Vishinsky, the most experienced crewmember, to lead the landing party. The ship has barely enough fuel to make the return journey — they do not have power for a scan before sending the party down to locate Sorenson and his team.
The Doctor and Sarah reach the base. They discover Braun’s almost mummified body on the ground. The base unit’s interior is dark. The Doctor surmises they are some months too late. Sarah goes back to the TARDIS to get his spectromixer so he can fix their position while he tries to restore the base’s power. When Sarah leaves, the Doctor discovers Baldwin’s body in the same mummified state.
Sarah makes it back to the TARDIS, entering as the armed landing party draws near. Vishinsky calls the probe ship, and Salamar orders the TARDIS brought to the ship and placed in quarantine. The landing party places a clamp on the TARDIS door, trapping Sarah, who has no idea what is going on. The police box is transmatted away.
The landing party spots Sorenson who, although acting a bit strangely, assures Vishinsky he is all right and his theory about Zeta Minor has proven correct. He found the vital discovery last night in Sector 5. Sorenson believes Baldwin returned to the base, suffering from fatigue. He leads the party to it. Vishinsky asks about the other six members of his expedition. Sorenson is evasive about the exact number and concedes they have lost some people, but the important thing is that the mission is a success. When they reach the base, the landing party find the Doctor working on the systems next to Baldwin’s body. Sorenson is startled to see the corpse. He says Baldwin has been murdered… just like the others.
Sarah finally finds the TARDIS doors unlocked and steps out to find herself on board the probe ship. Salamar tells her she is in orbit over Zeta Minor and a prisoner of the Morestrans. Salamar contacts Vishinsky and tells him to keep a careful watch on the Doctor while he questions Sarah. Salamar does not believe Sarah and the Doctor just “picked up” the distress signal, as Zeta Minor is so remote.
The ship lands on the planet near the base. Sorenson says they had only been working a few weeks when the killings began, always at night. Salamar believes it is the work of alien infiltrators and suggests the Doctor confess before he is subjected to interrogation. When Salamar is told there is no sign of life anywhere else, he concludes the Doctor and Sarah must be responsible and gives the order for their execution.
However, the Doctor and Sarah are escaping through the window of the store room they are held in, whose magnetic locks are weak due to the power loss. The moment they step out, they encounter a semi-transparent, monstrous figure, its outlines glowing red, reaching out for them.