The Captain and his Polyphase Avatron

The tracer detects that the second segment of The Key to Time is on the planet Calufrax - somewhere that The Doctor is less than enamoured with.

The TARDIS makes a very bumpy landing and when The Doctor, Romana and K9 leave to find the segment they soon discover that they are not on Calufrax at all. They have landed on a planet called Zanak instead.

As they explore The Doctor and Romana find that Zanak has an unusual civilisation that lives in perpetual prosperity. A strange band of people with mysterious powers known as the Mentiads are feared by the society, but The Doctor discovers that they are good people but with an unknown purpose.

The Doctor

The Doctor and Romana soon discover, when they explore the mine workings under Zanak, that this planet has in fact been hollowed out and fitted with engines so that it can transmat through space and materialise around others - such as Calufrax - to drain them of their energy and to plunder their mineral wealth, leaving them as shrunken husks held by gravitational forces in a ‘trophy room’ on Zanak.

Zanak is governed from a complex known as The Bridge by The Captain, whose body has been partially replaced with robotic parts following a near-fatal crash. This half-man half-robot though is merely a puppet controlled by his nurse - a projection of Zanak’s original ruler, the aged Queen Xanxia whose real body is held in stasis by a time dam.

Romana with a Guard

Queen Xanxia is using the time dam, powered by the forces generated in the trophy room from the plundered planets to keep the dam operational, to keep her body alive until her new, younger form has stabilised and has become permanent.

Unbeknown to The Doctor just as the TARDIS arrives The Captain prepares Zanak to ‘jump’ again - this time the target is a planet rich in PJX18 (quartz) which can be used to repair the engines. The planet chosen is Terra, otherwise known as Earth…

On learning the truth about Xanxia - that there is no amount of energy that will give her a permanent new form - The Doctor attempts to thwart The Captain’s plans and to stop Zanak from destroying Earth. In this he is aided by the Mentiads, a gestalt of telepaths amongst Zanak's indigenous population who are sensitive to the life force of the planets that are destroyed. Under The Doctor’s supervision they damage the engines by using telekinesis literally to put a spanner in the works.

Meeting the Mentiads

Meanwhile The Captain rebels against the nurse, by trying to assert his independence, and tries to free himself from the Nurse’s control but she kills him instead. The nurse herself is then destroyed by Kimus, a young member of Zanak’ civilisation whom The Doctor, Romana and K9 have befriended.

It is then that The Doctor realises that the second segment of The Key to Time comprises the whole of the planet Calufrax. On learning this 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.

Before departing, in the TARDIS, The Doctor helps to set up some explosives for the Mentiads, who then operate the detonator telepathically. Zanak’s control centre, The Bridge, is totally destroyed in a spectacular explosion.