‘You think you know us so well, Doctor. But we’re not abandoned. Not while we have each other.’ A story that’s superficially a Ballardian (or Stephen Wyattian, I suppose) black comedy about a global traffic jam. But, like the motorway, it has layers upon layers, of blind faith and true faith, and the difference between a belief and a lie, of sacrifice and redemption, and of the undercity rising up to inherit the (New) Earth.

‘They all go to the motorway in the end.’ If you wanted to look for religious metaphors, you wouldn’t have to look very hard. Below the motorway, lurking in sulphurous gas, are the Macra, waiting to consume the greedy or the liars or the thieves who try to cheat their way into the fast lane. Up above, the heavenly spires of the overcity and, I suppose, God – or at least the Face of Boe, making sure the world still turns even if he can’t intervene to save individual lives.

‘You are not alone.’ The Doctor’s bafflement as the New Earthers unite to sing The Old, Rugged Cross is palpable, and he’s the one who asks, ‘What if there’s nothing? Just the motorway, with the cars going round and round and round and round, never stopping. Forever.’ And yet, on this occasion, he’s wrong on just about everything. There is someone else: the Face of Boe has been giving his life for the people on the motorway. The survivors of the motorway are not the lost, they’re the saved, sustained both by the Face of Boe’s sacrifice and by their own belief in each other. There’s no dictator to overthrow or monster to defeat, even the Macra are mindless beasts, no longer ‘using humans as slaves and mining gas for food.’ Moffat will make a habit of these “broken system” stories, like Sherlockian logic problems. RTD makes it feel like revelation.

‘I lied to you because I liked it. I could pretend. Just for a bit, I could imagine they were still alive, underneath a burnt orange sky.’ The Doctor lies to Martha because even he needs to believe in something more. At the end he’s having to create a new lie to believe in, that the Face of Boe is wrong and ‘I’m not just a Time Lord. I’m the last of the Time Lords’. I think it’s really here that the image of the Doctor as the ‘the man who regrets’ really crystallizes, right at the point when we’ve been clued that he isn’t the last Time Lord at all. Tennant’s performance here, eyes filling with tears, barely able to contain his emotion, feels far beyond anything in Series Two. By contrast, Agyeman gets less to do than her first two episodes, but compellingly plays Martha’s own faith in the Doctor, which will prove crucial by the end of the series (and indignation when she realises he’s carting her round his old dating locations)

‘Oh, you should have seen it, that old planet.’ We get two descriptions of Gallifrey calling back to Susan’s dialogue in The Sensorites, setting up for an onscreen appearance in the finale. Taken with the return of the Macra this is a love letter to the 1960s series. But this is also the last in a trilogy of stories set around the year Five Billion with the Face of Boe, with the Time War thrown in matter-of-factly. This all comes at exactly the point when old fans were still reassured by the links to the past and new fans enjoyed being in on their own piece of the legend, and it feels like the ideal combination of myth-mentioning and myth-making.

‘That, and a bit magnificent!’ For me, Gridlock distils everything I love about the show. I love its eccentricity, its pace and relentless forward motion, its synthesis of source materials from comic strips to Coruscant. I love that the Doctor saves the world armed only with a screwdriver. I love that it creates an entire world from one re-dressed set and a couple of locations. I love its joy and optimism in the middle of the bleakness of a dead planet: there’s something that I think is very moving about the Doctor urging the frightened, forgotten people on the motorway to rise up. I love the guest performances, particularly Ardal O’Hanlon’s. I don’t think any other episode conveys what it might be like to be ‘the last of the Time Lords’ quite as poignantly. In the end, with so many to choose from, anyone’s favourite Doctor Who episode comes down to personal taste. This is my favourite.

Next Time: Daleks in Manhattan