The TARDIS lands on the surface of the moon. Its arrival is noted by Clinton Seymour, deep below the surface. He creates an air bubble round the TARDIS. The Doctor steps out and looks at the Earth, his intended destination, and wonders what pulled him off course. A black square opens near his feet and he climbs down some steps into an artificial gravity field. He is met by Roberta (Bobby) Goodman and Professor Dollund. The former is a young, pretty archaeologist and the latter is in charge of Survey Four. The Doctor’s psychic paper tells them that he is Doctor Smith of the Bureau of Alien Technology. The Doctor asks what they are surveying and is told that mineral shortages on Earth have led to prospective mines on the moon. They go to the control room and meet Clinton. He shows them a display where the automatic drills are shown approaching an unexplained object deep underneath the moon’s surface. Once the drills arrive a man called Henderson calls in from their position to say that they have opened up an enormous cavern. Within it is a chamber containing the object that had appeared on the display, but it is buried somewhere behind the walls.
Dollund and Bobby lead the Doctor to the elevator so that the three of them can see what the drills have uncovered. They descend to a tunnel that slopes down to a wide ravine. A narrow rock straddles the gap but as Dollund crosses it the ‘bridge’ crumbles under his weight and he only just scrambles across, ending up hanging from the edge of the ravine. The Doctor leaps across and pulls Dollund up. He tells Dollund that when the bridge was assessed for its load bearing capacity it was in a part of the moon that was subject to local gravity.. Now that the area has been opened up for more humans the control centre has imposed Earth-type gravity. Consequently, the bridge is only one sixth of the strength it had been earlier. He tells Bobby to get Clinton to reduce the local gravity to moon normal and when this is done she leaps the gap easily.
They continue their journey to the cavern where Henderson and Lisa Summerton are waiting for them. Henderson is a bluff scientist and Lisa is a young woman with close-cropped hair. The Doctor begins to dig away at one of the walls with his hands. The soft rock crumbles away to expose a human hand. Bobby realises that it is a stone hand and as the professor taps it the hand shears off. The hand is placed inside a plastic box and then the professor and the Doctor take it, via a longer route, back through the tunnels to the control room. When they take out the box to show Clinton they find that the box is now empty apart from some dust. Bobby calls to say that the hand has mysteriously reappeared in the wall of the cavern. The Doctor and Dollund make their way back down with some vacuum hoses. They use the hoses to clear the dust from the statue and find that it is an exact replica of the Doctor, apart from a crystal at its throat.
In the control room Clinton notices fluctuations in the readings down below, suggesting that a power source has been uncovered. As he listens to the description of the statue he looks around him and becomes aware that thick layers of dust are forming on all of the surfaces and that the hand has suddenly reappeared near him. He radios down to tell the others this news when his call is cut off. He is horrified when the hand jumps at him and tries to cover his face. He fights it off and smashes it to dust with a chair as it leaps again. Just then, the door bursts open and the Doctor and Henderson rush in. No sooner have they got their bearings than Lisa calls to say that the statue no longer looks like the Doctor. It is now a copy of Bobby.
Across the galaxy is the desolate planet Karagula. In a giant building resembling a medieval cathedral is a huge chamber. Three members of the Darksmith Collective are gathered. Brother Talen announces that the Crystal has been found and Sister Hellan says it can be located within Mutter’s Spiral. The exact location is being traced. High Minister Drakon is pleased because centuries of waiting are almost over for the Darksmiths and they now have the chance to regain one of their most valuable artifacts.
The statue in the chamber suddenly loses all of its distinguishing features, becoming a blurred figure. It edges forward and tries to smother the professor. Lisa and Bobby drag him away and the three of them run out of the chamber as another indistinct figure emerges from the walls. The Doctor and Henderson race to meet them in the tunnels, just as more shapes come out of the walls. As the five of them escape up the tunnels to a metal access passage the lights flicker and die; the generator has failed. Lisa and Henderson head off to the generator room to try and effect some repairs. The room and generator are choked with dust and as they start to vacuum the machinery they realise that the dust is coalescing into human shapes. Clinton calls to ask how things are going but all he hears is static. The Doctor and Bobby, who have been in the control room, run down to investigate. When they get there they find the generator room covered in settled dust. Beneath the dust are two bodies: Henderson and Lisa. The Doctor checks them and finds that both are unconscious. They haul them out into the corridor and Bobby starts to clear their airways of dust while the Doctor restarts the generator.
Lisa and Henderson are soon well enough to be helped back to the control room but the way is blocked by more dust creatures. The Doctor radios Clinton to shut off the sections behind them as they progress towards the control room. He also wants the ventilation set to maximum. The dust is forming into arms and hands that grab at them until a blast of air from the ventilators blows it away and they run for safety. When they arrive Clinton tells them he sent a distress signal but has not been able to receive any confirmation. Then a pile of dust in the room forms into a humanoid figure that strangles Clinton just as the door is pushed open and more dust creatures enter.
On Karagula the Darksmiths have used Nestene and Gundan technology to manufacture a huge robot called the Agent. The High Minister announces that the robot will be sent to retrieve the Crystal and fulfill a centuries old contract that was left incomplete by Brother Varlos when he stole the Crystal.
Bobby saves Clinton’s life by fighting off the dust creature. The others notice that one of the dust men has the Crystal from the chamber around its neck. The Doctor realises it must be the Crystal that is animating the dust and sets his screwdriver to negate the signal.
The Darksmiths prepare a supra-lightspeed pod to take the Agent to the Crystal once its coordinates have been finalized. They hope that the Agent will also discover Varlos and bring him to justice. The Crystal is finally located and the Agent is ordered to retrieve it with no respect for any lives that may have to be lost in doing it.
The Doctor uses the screwdriver and the dust creature explodes. The Doctor picks up the Crystal and puts it into his pocket. On Karagula the Darksmiths are disappointed to find the Eternity Crystal has stopped sending a signal. However, they can trace its last location and consult their charts to see that it was on the Earth’s Moon. They send a message to the Agent warning it that it may be facing a powerful enemy, possibly Brother Varlos, but certainly someone capable of switching on or off the power of the Crystal.
The Crystal’s emissions have actually been blocked by putting it into a stasis casket in the TARDIS. The Doctor decides to leave the Moon but first he calls in on his recent friends in Survey Four to make sure they are alright after their traumatic experience. When he returns to the surface after saying his goodbyes he is horrified to see that the TARDIS has gone. A number of vehicles from the Moon Base have arrived at Survey Four and some of them are still there. However, from tracks on the moon’s surface the Doctor can see that the TARDIS has been loaded onto a moon buggy and transported away. He gets a lift on another buggy to the Spaceport. He traces the TARDIS to Hangar 7 but his ship is not there. Instead, there is a square of dust where it once stood. Worse, there is a sophisticated robot made of metal and plastic standing in the hangar. The Agent is waiting for him.
The Agent addresses the Doctor as Brother Varlos and says he has been tried in his absence and sentenced to death…