The TARDIS makes a bumpy landing on the Moon in the year 2070 and, dressed in spacesuits, the Second Doctor and his companions Jamie, Polly and Ben venture outside and enjoy the low gravity environment. Jamie is injured while they fool around.
The only building on that part of the moon’s surface is the Moonbase, a weather tracking and managing station which is staffed by an international crew managed by the bullish Hobson. They are using the gravity machine the Gravitron to control the Earth’s weather. When the time travellers arrive, the Doctor ingratiates them in the usual manner and is on hand to do some detective work because some of the crew have been collapsing in comas with a strange virus that spreads throughout the body. International Space Control responds to the crisis by quarantining the Moonbase. The station doctor, Evans, was the first one to get the disease and in a delirium talks of a silver hand but immediately dies of the disease.
Something is definitely amiss because another of the Moonbase crew, Ralph, has disappeared in the food stores, and they have also noticed the Moonbase radio transmissions are being monitored from somewhere else on the moon. Jamie has been placed in the sickbay where, feverish and delirious, he begins mumbling about a “Phantom Piper”, a figure which appears to McCrimmon before he dies. While attending to Jamie, Polly sees a large figure leave through the door. When Hobson, The Doctor, Ben, John and Nils arrive to collect Evans' body it has disappeared. They then leave to investigate where this 'piper' is. Polly goes to get some water, and Jamie wakes up to see the 'piper' advancing on him.
The 'piper' ignores Jamie, as he doesn't have the disease, so he steals another patient and leaves. Polly comes back in just as the figure is leaving and recognises it as a Cyberman, and the Doctor realises their old enemies are stalking the Moonbase and taking the patients' bodies. Hobson brushes away the cyber-story, believing they died out years ago. He gives the Doctor 24 hours to discover the cause of the virus, or else he leaves the moon.
The Moonbase with animated missing eposodes signed by Anneke Wills & Barry Noble