Up to now some of Baker and Martin’s more off-the-wall ideas have been reined in presumably by Terrance Dicks and Christopher Barry, meaning that The Mutants hasn’t been quite as disconcertingly weird as The Claws of Axos. That changes in this episode, which sees the Doctor plunging into a radioactive crystal cave to satisfy his thirst for knowledge (he should be careful about that) in a sequence that goes on forever and throws every video effect available in 1972 at the screen including slow-motion, electronic lightning and strange blobs, and sparkly haloes around everything. It’s as trippy as the inside of Axos, and the weirdest thing we’ve seen so far in a season that’s largely toned down some of the tartrazine excesses of Season Eight.

The background to this sequence is the Doctor’s discovery that the stone tablets he’s been sent to deliver to Ky represent the seasonal variations of Solos. This is very 1970s (fascination with ancient Mayan calendars and prophecies and all that jazz), as is Professor Sondergaard, who as a young man eschewed the Marshal’s cruel treatment of the Solonians and went native, with the fully hippy bead look. As with all 30th Century scientists he sports a European accent and complains endlessly about his rubbish equipment. The big reveal: ‘The Solonians are meant to mutate’ to cope with the planet’s slow orbit around its sun, is very Out of the Unknown, a neat kink in an otherwise fairly standard rebels vs imperials serial.

I also liked the logical reasons for everyone to begin to reconvene on Skybase – Varan, insane with bloodlust and vengeance, captures Jo and Ky and uses them as human shields in a kamikaze attack on the overlords. Sondergaard and the Doctor realise they need to get to Jaeger’s equipment to analyse the mysterious crystal they’ve found. The Marshal just wants to accelerate the terraforming of the planet with a rocket attack from orbit. Al this gives some momentum towards the final couple of episodes.

I also quite liked some of the ambition in the effects work. As so often the case, the show’s reach exceeds it grasp. To represent the earthquakes on Solos, Barry points his camera towards some sort of mirror material that wobbles to represent the tremors. It’s a bit more imaginative than just shaking the camera and expecting the actors to stagger about, but it doesn’t quite work because everyone is a bit distorted in the mirror. The cliffhanger is astonishing in theory – the Marshal blasts a hole through Skybase’s hull, Varan is sucked into space, and everyone else has to cling on for dear life. In practice, this is an effect even a 1970s movie would struggle with and so you’re left understanding what they were trying to do rather than buying it – but even so, combined with the launch of Jaeger’s rockets it’s a hell of an ending.

Next episode: The Mutants – Episode Five