The Claws of Axos

From http://www.bbc.co.uk/doctorwho/classic/episodeguide

An approaching alien spaceship is detected on monitoring equipment at UNIT HQ, where the Brigadier is entertaining two visitors - Chinn, a civil servant making a security inspection, and Bill Filer, an American agent sent to discuss the threat of the Master. The ship lands in England and the UNIT team, joined by Hardiman and Winser from the nearby Nuton power station, meet its occupants: beautiful golden-skinned humanoids called Axons.

The Axons claim that their ship, Axos, is damaged and that they need time in which to repair it. In return, they offer Axonite, a substance that can cause animals to grow to enormous sizes and thus end food shortages. The Doctor is suspicious, and rightly so: Axos, Axonite and the Axons - whose true appearance is hideous - are all part of a single parasitic entity brought to Earth by the Master to feed on the planet's energy.

The Doctor manages to materialise his TARDIS, with the Master on board, at the centre of Axos. He offers to link the two ships together to make one giant time machine, on condition that Axos in return helps him to take revenge on the Time Lords for exiling him to Earth. This is merely a trick, however, and Axos is locked in a time loop from which it can never escape.

The Doctor returns to Earth in the TARDIS, where he reluctantly admits to the Brigadier that the Master may also have escaped.

4 out of 103 (3.9%) raters say this story requires a previous story.