We’ve known from literally the start of this serial that the Master was going to show up at some point, the mystery was when and where. If you don’t know what’s coming, the cliffhanger of the last episode might suggest that he’s been lurking in the “Primitive” city, waiting for Jo to be brought before him. But, in fact, he’s disguised himself as the Adjudicator. Surprise!

Fortunately for the Master, the Doctor’s gone to the ruins to rescue Jo, and has discovered the remnants of a fallen civilisation that has reverted to savagery from technological advancement. It’s a neat sequence, using the deteriorating sophistication of the murals to tell the story of the decline, and it’s quite fun that the “Primitives” have a caste of giant-brained but almost blind priests directing them like silent movie grand dames, and a tiny, withered leader lurking in a cupboard (its face always reminds me of Pertwee’s in the Season 11 titles).

There are lots of nice things about the episode: the Doctor and Jo’s journey into the ruined city adds a new element to the story at the point where the squabbles between IMC and the colonists is getting tired, and the arrival of the Master gives the second half of the story a boost of melodrama. Delgado beautifully plays the Master’s evident surprise at the Doctor’s arrival, and he gets some great one-liners (‘It’s always the innocent bystander who suffers eventually’), as does Pertwee (‘Forged of course,’ says the Doctor, examining the “Adjudicator”‘s credentials – a line that gets repeated in The Five Doctors). The IMC crew getting drunk to celebrate the Adjudicator’s decision in their favour is a very believable touch. And Norton, to no-one’s surprise, is finally unmasked as an IMC spy. It’s an entertaining episode that reinjects some energy into a serial that was flagging.

Next episode: Colony in Space – Episode Five