The sinking feeling when Robert Sloman’s name appears in the opening credits is misplaced. There’s none of the archness and whimsy that disfigured his last script, The Time Monster. Instead, we’re presented with something that skirts dangerously close to polemic as Professor Jones and Jo Grant take a stand against the diesel pollution threatened by Global Chemicals’ new factory in South Wales. This has a sense of purpose and passion beyond pretty much anything the show has previously offered, and the result is one of the most instantly compelling episodes ever.

What I particularly like is the degree of self awareness in the script. While Professor Jones and Jo are clearly positioned as being on the right side, there’s a nod to how easy it is for them to to be progressive in all their middle class, Nobel Prize-monied comfort, while the unemployed miners of Llanfairfach can’t live on fresh air and moral superiority, and need the jobs offered by Global Chemicals. It’s not kitchen sink drama, but the level of social realism, the visuals of a miners’ picket line and the psychedelic Wholeweal community signs, make this feel a lot closer to home than the country houses, scientific research facilities and military installations that have mainly represented contemporary Britain through the last four seasons.

Social realism to an extent: you wouldn’t get the line ‘The fellow’s bright green, and dead!’ in Cathy Come Home. Nor the series of unfortunate events that befall the Doctor on his trip to Metebelis 3, the famous (and literally) blue planet of the Acteon Galaxy. Nor the strange mind control device that factory boss Stevens has secreted in his office. There’s a mystery here – and one the audience don’t yet understand the nature of. We haven’t been shown an alien spaceship arriving, a dinosaur lurking underground, or the Master plotting. Usually by the end of a Pertwee first episode the menace is relatively obvious, but so far there’s not a giant maggot or BOSS to be seen.

Instead, the episode invests in the regular characters in a way that reflects the level of care and attention the series is receiving. The first scene with Jo (cosplaying the fifth Doctor) and the Doctor immediately establishes a new tension between them. The Doctor, once again, has all of time and space at his fingertips, and offers it to Jo. She has more earthly concerns: saving this planet. The Doctor tells her, ‘You choose for yourself.’ Shockingly, she chooses to leave him, and even tells the Brigadier that she will resign from UNIT. ‘The fledgling flies the coop,’ he reflects sadly. It’s not even her last episode, and Katy Manning’s already had a leaving scene that’s better than any companion since Ian and Barbara’s. Later, her first encounter with the stunningly handsome Professor Jones consciously evokes her introduction to the Doctor in Terror of the Autons, helping to establish for anyone paying attention that this is going to be Jo’s last bow. This is an absolutely excellent first episode.

Next episode: The Green Death – Episode Two