Doctor Who: The Pirate Planet

As the Doctor, Romana and K9 head for the planet Calufrax in search of the second segment of the Key to Time, they are in for a surprise when the TARDIS brings them to Zanak, an entirely different planet occupying the same space coordinates. Did Romana take a wrong turn when she navigated by the book? No, the tracer registers that the segment is near. Nothing is as it seems in this story by Douglas Adams (The Hitchhiker's Guide to the Galaxy). The populace of Zanak lead affluent lives, thanks to the mysterious mechanizations of the Captain. He regularly announces a new age of prosperity, and most of the people accept the extraordinary turn in their fortune without further examination. Little do they imagine that their benefactor - hidden on a forbidden mountaintop - is a roaring space pirate whose best friend is Polyphase Avitron, a cyber-parrot that brings death to anyone who displeases its master. All these elements will come together to reveal the greatest crime in the galaxy and the second segment of the Key to Time as well.

The tracer detects the second segment on the planet Calufrax. The TARDIS makes a bumpy landing, and the Doctor and Romana soon discover that they are not on Calufrax at all. They are in fact on Zanak, a planet that has been hollowed out and fitted with engines so that it can transmat through space and materialise around others - such as Calufrax - to plunder their mineral wealth, leaving them as shrunken husks held by gravitational forces in a 'trophy room'.
Ostensibly in charge is the Captain, whose body has been partially replaced with robotic parts following a near-fatal crash, but he is merely a puppet controlled by his nurse - a projection of Zanak's original ruler, the aged Queen Xanxia.
Xanxia is using time dams, powered by the forces generated in the trophy room, to keep her body alive until this new, younger form has stabilised. The Captain prepares Zanak to 'jump' again - this time with Earth as its target.
The Doctor attempts to thwart his plans, calling on the assistance of the Mentiads - a gestalt of telepaths amongst Zanak's indigenous population - to sabotage the engines.
The Captain rebels against the nurse, but she kills him. The nurse herself is then destroyed by one of the Zanak natives whom the Doctor, Romana and K9 have befriended.
The Doctor realises that the second segment comprises the whole of the planet Calufrax. He contrives to drop the compressed husks from the trophy room into a space-time vortex created by the TARDIS in the centre of Zanak, and is then able to pick up and convert the segment at his leisure.

Written by Douglas Adams
Directed by Pennant Roberts
Costumes by L Rowland Warne
Film Editing by John Dunstan
Incidental Music by Dudley Simpson
Produced by Graham Williams
Script Editing by Anthony Read
Title Music by Ron Grainer and the BBC Radiophonic Workshop, arranged by Delia Derbyshire
Visual Effects by Colin Mapson

There are some excellent location scenes filmed at the Dan-yr-Ogof caves in Powys, Wales.