As the masterminds behind the Peruvian temples (at least according to the Doctor), the Exxilons are another of those Arthur C Clarke’s Mysterious World aliens the series was obsessed with in the early 1970s. It feels like we’ve heard the back-story of the living city of the Exxilons several times before already, and the possibility of the Doctor and Belal braving an Indiana Jones race through Inca traps is quickly put to bed when the puzzles that have confounded so many turn out to be tracing a maze, and (in one of the dumbest cliffhangers ever) avoiding some floor tiles.

All the bits with the Exxilons are fairly dull. Belal’s surprisingly articulate info-dump about their backstory goes on for ages, and the Crystal Maze challenges he and the Doctor face (three wrong turns and it’s an instant lock in) while strangely prescient of the Dark Tower sequences in The Five Doctors, are a typical Terry Nation device for padding out the script (how many pages? That’s ok, bung in another coffee break brain-teaser).

For the first time in the colour era, the Daleks are the most interesting thing about this. When they’re on screen, Briant keep finds interesting, even iconic things to do with them. For example, the location film “root” attack on one of the Dalek guards, culminating in its dome exploding, is spectacular, and the inspiration for the most eye-catching of all the Target book covers. Even the studio work is pretty strong (including another “root” versus Dalek punch-up, weirdly cheered on by the Doctor). But for all Briant’s efforts, he can’t quite turn this sow’s ear into a silk purse.

Next episode: Death to the Daleks – Part Four