The Doctor takes Ace to present day Perivale so that she can revisit old friends. Most of them however have been transported by cat-like Kitlings to the planet of the Cheetah People, a race with the power to teleport through space. Ace is transported and joins up with two of her friends, Midge and Shreela, and a boy named Derek.
The Doctor follows and encounters the Master, who has drawn him into a trap to try to gain his help. This planet gradually transforms its inhabitants into Cheetah People - an influence to which the Master himself has fallen victim - while they in turn, through the savagery of their actions, cause the planet to move ever closer to total destruction.
Midge is overcome by the planet's influence, and the Master uses him to teleport to Perivale. Ace, who has developed an affinity with a Cheetah woman called Karra, gains the same ability and takes the Doctor and the others back as well. The Master causes Midge's death and kills Karra, who has followed them all to Earth. Then, succumbing to the influence of the now-disintegrating Cheetah planet, he drags the Doctor back there.
The Doctor refuses to fight him, however, and is transported back to Earth. He rejoins Ace, and they head off for new adventures.
Anthony Ainley (The Master), Julian Holloway (Paterson), Lisa Bowerman (Karra), Norman Pace (Harvey) , Gareth Hale (Len) , William Barton (Midge), Sakuntala Ramanee (Shreela), David John (Derek), Sean Oliver (Stuart) , Kate Eaton (Ange) , Kathleen Bidmead (Woman) , Adele Silva (Squeak) , Michelle Martin (Neighbour) ., Adele Silva (the girl)
Writer - Rona Munro, Director - Alan Wareing ,Assistant Floor Manager -Stephen Garwood, Assistant Floor Manager - Leigh Poole Costumes, Ken Trew Designer - Nick Somerville Incidental Music - Dominic Glynn Make-Up - Joan Stribling OB Cameraman - Paul Harding OB Cameraman - Alan Jessop Producer - John Nathan-Turner Production Assistant - Valerie Whiston Production Associate - June Collins Script Editor - Andrew Cartmel Special Sounds - Dick Mills Stunt Arranger - Paul Heasman Stunt Arranger - Tip Tipping Title Music - Ron Grainer and the BBC Radiophonic Workshop, arranged by Keff McCulloch Visual Effects - Malcolm James