Made before Horror of Fang Rock, this is new producer Graham Williams’ first Doctor Who serial. After all the fuss kicked up by Mary Whitehouse about violence during Hinchcliffe’s run, Williams had been instructed to tone down the horror. I think that’s instantly clear. The opening scene on a spaceship navigating the asteroid belt is apparently crewed entirely by students of the Rogin school: 1970s workers transported to the year 5000, and still griping about how dull their jobs are, and how often they’ve been overlooked for promotion. Equally, the crew of the Titan base can’t wait to go home, have broken out the bubbly and fruit bowl. The new series has made an art out of making the future feel familiar in precisely this way.

The Doctor and Leela’s TARDIS scene (back in the traditional Number 2 console room) is also played more broadly than last season. Back in her skins for the first time since The Robots of Death, Leela feels like the same character, but somehow gentler, with more focus on her uncanny savage instincts (she instantly realises the Titan crew have been possessed) than her knife violence. There’s some fun banter about Leela parroting the Doctor, and then when the Doctor himself falls under the Swarm’s influence, some funny business of him being unable to speak properly (‘a bot of a shick’) and walking into walls, before goggling at the possessed spaceship crew. The show hasn’t done anything this broad since the late 1960s, and coming off the back of the brutal Horror of Fang Rock it’s even more of a jarring contrast than had it followed the black comedy of The Talons of Weng-Chiang.

It also makes for a strikingly effective contrast with the Doctor’s sudden switch to host for the Nucleus, which Baker plays absolutely straight. Picking up a gun to hunt down Leela – the ‘reject should be destroyed’ – and gently trying to coax her out of hiding is very sinister indeed. The creeps are only undermined slightly by the sudden and disastrous collapse in the show’s design standards. The possessed are identified by enormous, Underwater Menace style bushy eyebrows and scales, and they’re walking through sets that are typical late-1970s BBC sci-fi on a budget. Control panels are covered in colourful lumps of plastic; the signage is in Westminster machine-readable typeface, the walls wobble (especially when Michael Sheard presses the emergency exit, or should I say Imurjinsee Egsit, control), and everything is lit like a supermarket. Compared to the previous season opener, The Masque of Mandragora, the studio work in this is dreadful. Which is a shame, because the spaceship models, and the flight through the asteroid belt, are way more impressive than the Helix.

Next episode: The Invisible Enemy – Part Two