The TARDIS hurls its occupants around but finally materialises on a barren, cratered landscape. Polly thinks that the Doctor has succeeded in steering the ship to Mars, but Ben recognises their location as the Moon. The Doctor reluctantly agrees to a brief 'shore leave'. Once outside, the spacesuited friends enjoy the sensation of near weightlessness, jumping huge distances over the Moon's surface. Jamie loses control and falls unconscious near to a domed lunar base. Two men appear and take him inside. The Doctor, Ben and Polly follow. 

The base is Earth's weather control centre, manned by an international team, in the year 2070. The base is currently being afflicted by a strange plague which has already struck down the doctor, Evans (Alan Rowe). Jamie is taken to the medical unit and Polly visits the young Scot whilst Ben and the Doctor are quizzed by the base commander, Jack Hobson (Patrick Barr). The Doctor learns of the base's Gravitron device, which control the Earth's tides, and realises that the beam from this was responsible for the TARDIS's erratic landing. One of the Gravitron operators collapses with the plague and the beam drifts across the Earth's surface, causing a hurricane to threaten the Hawaiian islands. Space headquarters on Earth contacts the base and is told of the plague. The base is placed under quarantine. Nils Jensen (Michael Wolf) reports that the communication with Earth was monitored. Unbeknown to the men on the base, a saucer shaped craft lies in a nearby crater monitoring all their frequencies. Hobson makes a morale-boosting speech to his crew while Ben and the Doctor are taken to see Jamie. 

Jamie is delirious, begging his friends to protect him from a phantom piper who comes to claim the bodies of the McCrimmons. The Doctor examines Evans and the other two plague sufferers and announces that their symptoms do not indicate a virus at all. Ben is sent to help the understaffed Hobson and joins Ralph Adebayo (Mark Heath) in taking stock of the base's food supplies. A silver figure (John Wills) emerges from the shadows and incapacitates Ralph with a spark of energy from its hands. By the time Ben reaches the spot the silver figure has vanished and so has Ralph. 

Evans cries out 'The silver hand!' and dies. The Doctor goes to tell Hobson what has happened and arrives in time to hear Ben explaining about Ralph's disappearance. Hobson decides to inspect Evans' body, but they arrive back to find Polly screaming. She has spotted something slipping out of the medical unit. The Doctor, Nils and Bob Anders (Edward Phillips) conduct a search but find nothing. Hobson is furious when he discovers that Evans' body is missing. His second-in-command Roget Benoit (Andre Maranne) rushes in to warn him that the Gravitron is out of alignment again. Before leaving, Hobson orders the Doctor to find the body. 

With both the Doctor and Polly temporarily absent, the silver figure appears in the medical unit and moves toward Jamie, who thinks the phantom piper has come for him. The creature picks up one of the two infected men. Polly returns to see it disappearing through a door. Her screams bring the Doctor, Ben, Hobson, Sam Becket (John Rolfe) and Nils running in. She tells them that she saw a Cyberman. Hobson is sceptical and gives the travellers twenty four hours to cure the plague and find the missing bodies. 

The Doctor uses the base's medical equipment to investigate the plague. In the control centre Hobson and Benoit are meanwhile struggling with more problems caused by the seemingly malfunctioning Gravitron. Benoit is keen to shut the machine down but Hobson and the Earth-based space controller Rinberg (Denis McCarthy) insist that it is needed to help deflect a hurricane away from the coast of America. As the scientists make thorough checks of the systems the Doctor bustles around taking samples of various items for testing. 

A Cyberman enters the medical unit and attacks Polly with an energy spark. While she is unconscious it takes away the remaining plague sufferer, carrying the man out of the base through a hidden hatch cut into the dome. The resulting temporary dip in air pressure is registered in the control room. The Doctor returns to find Polly collapsed. 

The scientists discover that the fault in the Gravitron is due to damage to one of the external probe control antennae. Hobson is convinced that the four travellers are guilty of sabotage. He sends Jules Faure (Victor Pemberton) and Franz Schultz (Barry Ashton) to repair the damage and is about to go and confront the Doctor when Ben rushes in to tell him of the latest disappearance. He storms into the medical unit in time to hear the Doctor declaring that he has made a discovery. The Doctor later admits to Polly that he has still found nothing and was bluffing. 

Jules and Franz are working on the damaged antenna when they are attacked by two Cybermen (John Wills, Peter Greene). 

Another man collapses while drinking a cup of coffee prepared by Polly and the Doctor realises that the base's sugar supply has been contaminated by the Cybermen with a neurotropic virus that attacks the nervous system. Hobson remains dubious, but a search of the medical unit uncovers a Cyberman hiding beneath a blanket on one of the beds. 

A crewman attempts to attack the Cyberman but is gunned down. A further Cyberman enters and tells Hobson that the bodies of the sick and dead men are needed for conversion. Jamie - who has not been exposed to the virus and has a head injury - is useless to them, but the remaining plague sufferers are carried away. Ben and Polly remain in the medical unit with Jamie while the Doctor and Hobson are taken to the control room. The Cybermen intend to use the Gravitron to destroy the Earth with extreme weather. 

Jamie, now recovering from his head wound, is told by Polly and Ben of the threat posed by the Cybermen. Polly suggests fighting back by using a solvent to attack the plastic components in the creatures' chest units. 

Three of the Cybermen's prisoners, including Evans and Ralph, are brought into the control room. They have been fitted with control helmets and move like zombies. They take control of the Gravitron in preparation for carrying out the Cybermen's plan. The Doctor wonders why the Cybermen do not control the Gravitron themselves. He also discovers that by using the base's radio transmitter he can affect the signals being sent to the controlled men. Hobson knows that Rinberg will send a rescue rocket to the base if it fails to react to his radio messages, but he conceals this fact from the Cybermen. 

In the medical unit Ben has shown Polly how her solvent concoction can be sprayed at the Cybermen using fire extinguishers. They head for the control room, accompanied by Jamie. The Doctor uses the radio to jam the signal to the zombies in the Gravitron area. Under cover of the distraction Ben, Polly and Jamie burst in and spray the Cybermen with the solvent, which causes foam to spew out of their chest units as they die. The controlled men are taken to the medical unit while the remaining crew members attempt to regain control of the Gravitron. 

In their spaceship the remaining Cybermen realise that their initial plan has failed. The prepare for a massive attack on the base. 

Benoit has ventured outside looking for Jules and Franz. All he finds are two empty spacesuits. Ben rescues him from an attack by a Cyberman - killing it by throwing a bottle of solvent at its chest unit - and the two men return to the base. Hobson decides to secure the place by lowering armoured doors. Nils locates the Cybermen's ship on the radar approximately three kilometres away. The group look on in horror as an army of Cybermen (John Clifford, Ronald Lee, John Levene, Barry Noble, Sonny Willis, John Wills, Reg Whitehead, Peter Greene, Keith Goodman. Voices: Peter Hawkins) advances across the Moon's surface. 

The Cybermen contact the base by radio and call on it to surrender. Hobson tries to get a warning to Earth but the creatures use their weapons to destroy the base's aerial. Nils uses a solar telescope to keep a lookout for the rescue rocket from Earth. The Cybermen are also aware that the rocket is on its way and make plans to destroy it. In the medical unit, Evans rises from his bed and places the control helmet back on his head. Following the Cybermen's orders he slips back into the control room, enters the Gravitron area and knocks out the on-duty operator Joe Benson (Alan Wells). He then uses the machine to prevent the landing of the rescue rocket, sending it out of control toward the Sun. The crew see Evans at the controls holding one of the Cybermen's guns. 

Ben and Jamie construct a barrier of benches and chairs to prevent the other controlled men from breaking out of the medical unit. 

The Cybermen again call on the base to surrender and then fire a laser weapon which blasts a small hole in its dome, causing the air to start rushing out. Benoit and Hobson manage to patch the hole with a drinks tray and Nils engages the reserve oxygen supplies. Evans is now unconscious and is taken away to be locked up. 

As more Cybermen spaceships land nearby the Doctor realises that he can use the Gravitron against the invaders. He adjusts the angle of the machine and when the Cybermen fire their laser weapon again the beam is deflected. The Doctor then has the Gravitron directed at the Moon's surface, sending the Cybermen and their ships hurtling off into space. The crew celebrate, but when Hobson turns to thank the Doctor and his friends he discovers that they have gone. 

On their way back at the TARDIS the travellers see a shooting star, and the Doctor hopes that this will be the last they see of the Cybermen. The ship takes off and the Doctor uses the time scanner to provide a glimpse of what lies ahead. Polly points to the screen in fear as a giant claw appears.

_{Synopsis from Doctor Who: The Second Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}