‘Never mind Planet Zog. A party in the 1920s, that’s more like it.’ I bet a lot of viewers were expecting a whole series like this when Catherine Tate was announced as a regular. It’s easily the funniest new series episode to date, gleefully pastiching the conventions of countless Christie TV adaptations (the classic cars), name-dropping her novels, poking fun at British stereotypes and using Tate in the most Catherine Tate Show way possible. The end result is a glorious, summery and charming, literary comedy, The Androids of Tara of the 21st Century.

Like any Christie novel, there are shades of darkness as well as light comedy. There’s fading anger about the way gay men were forced, by law, to hide their feelings, and how women had to hide their indiscretions for shame while men like the (unseen) Archie Christie paraded their sexual triumphs on their arm. Donna’s beautiful heart to heart with Christie gets to the nub of this, without ever becoming a polemic.

But essentially, Gareth Roberts is doing this like one of Christie’s early murder romps: it’s more The Secret of Chimneys than The Murder of Roger Ackroyd, with Donna taking the role of Lady Eileen “Bundle” Brent or perhaps Tuppence Beresford to the Doctor’s Tommy (the original Partners in Crime). It’s packed with adorable moments, the best of which include Lady Eddison’s flashback to a scene we saw a couple of minutes before (which I suspect is mocking modern TV detective shows like Elementary); Donna helping the Doctor recover from cyanide poisoning (a scene replayed for straight in Torchwood Series Four); Colonel Curbishley flashing back to a saucy evening of yesteryear, and a spoof of the novel Human Nature (the Vespiform ‘made himself human, to learn about us’). The end result is a “celebrity historical” that pokes fun at the idea of celebrity historicals, and has a healthy iconoclasm at precisely the moment that Doctor Who was beginning to believe in its own myth. Given what’s coming next, you could almost call it the end of an era.

Next Time: Silence in the Library