I’m coming to the conclusion that Doctor Who isn’t very good at endings. This is easily the weakest episode of the story, with far too much wandering about, the Doctor solving everything with a massive gun, and then forgetting all about it. It’s very “that’ll do”. And it’s bizarre that the De-Mat gun should be the solution to the Sontaran threat, when constructing it relies so much on secret presidential knowledge. If there had to be a particularly boring and secret super weapon, it might have been a better pay off to the long game with the Vardans rather than improvised off the cuff.

Leela’s leaving is equally cursory. She spends more time with Idas in Underworld than here with Andred. I wish Williams had listened harder to Louise Jameson, rather than assuming he could persuade her to stay and botching the exit of such a distinctive character (and, worst of all, he didn’t even learn his lesson the following year). Even at the last minute, there were better options than partnering her up with Andred: have the outsiders beg her to lead them, or even Borusa ask her to be the new Castellan after the last one resolutely failed to defend Gallifrey. It’s a bitter pill, given the effort Louise Jameson put in over the last 18 months.

It’s not all bad. It’s still quite witty: I love the attempts to make the TARDIS an Escher-like environment. If he could look past Tom Baker’s comedy trip, Christopher H Bidmead would be proud of the scenes where every door on different sides of a corridor leads to the same room, or when the Doctor keeps going through the same storage room as he descends through the levels. I’m not bothered that there’s no effort made to add roundels or dress the location like the inside of a TARDIS – but neither did the boot cupboard in The Masque of Mandragora, or (presumably) the cricket pitch just offscreen in Castrovalva.

I’m intrigued, too, when the idea of a Time Lord companion for the Doctor took shape. Rodan is so close to the first Romana (and, understandably, Mary Tamm plays it similarly to Hilary Ryan), and gets so many companion type things to do (helping the Doctor to foil the Sontarans, and even getting a sort of handover scene with Leela) that it’s almost a surprise that she doesn’t rush forward and leave with the Doctor at the end. I wonder if Ryan was asked and turned it down, or whether Romana was developed as a result of how well Rodan worked here.

In general, the final two episodes of The Invasion of Time are where it’s at its weakest. The surprise reveal of the Sontarans is a brilliant cliffhanger, but their threat is underdeveloped in the final two episodes, and they end up being as lightweight and flimsy as the Vardans. Borusa practically disappears for the middle of the story, so we’re denied the pleasure of Tom Baker and John Arnatt’s double act for a long stretch. The TARDIS chase scenes are a joyful mess. It’s clearly a product of desperation, but it works so much better than Underworld that it’s much easier to look past the general shabbiness. Season 15 seems to have been as nightmarish to make as Hartnell’s third year. It’s easily the least consistent run of the 1970s. Underworld and bits of The Invisible Enemy aside I’ve largely enjoyed it, but I’m looking forward to the step up in quality around the corner.

Next episode: The Ribos Operation