The current series of Doctor Who has been pretty variable in quality, often thanks to the very presence of Catherine Tate, who has had moments but, as a rule, I can’t stand – the announcement that she was a permanent fixture was almost enough to make me bail out completely – but last night’s penultimate episode of the series was something else. As if it wasn’t already apparent, there will be spoilers here after the break. Those who haven’t seen it and want to know what happens, there’s a good synopsis here.

The big crossover between the three previous series’ of Who and its two spin-offs, as well as the now-annual return of [insert major Who villain from the past here] (an awesome incarnation of Davros, in this case) was well-known and the major talking point beforehand, but OH MY GOD at that ending.

In these days of the Internet when every major plot twist leaks weeks and months before they happen (least of all one with as much mainstream coverage as Who), if the fact that Tennant is leaving already and will actually regenerate into someone else at the beginning of next week has been kept under wraps, the BBC has pulled off both one of the great TV rug-pulling acts in history. Not even a rumbling of a search for a new actor. Just wow…

Of course, this is the last series written by Russell T Davies, who has a quota of three gay characters and a contrived deus ex machina in every episode (the Torchwood characters fulfil the first criteria), and if anyone can write a silly way out of this he can. The fact that where we left our heroes is one second out of sync with the rest of the universe and they made a point of showing The Doctor’s disembodied hand in the episode both leave potential ways out, as do – please no – a few hints as to Donna’s future. Throw in a sonic screwdriver, the fact that this series has already had two alternate realities, and a bit reverse-polarity neutron flow technobabble and they’re quite capable of throwing it all away and getting back to the status quo.

But let’s assume, as even the BBC’s Radio Times is doing, that all is as it appears at the end of the episode. If they’re not going to undo this week’s brilliant work, it’s not only very brave to do this before the end of a series when such bombs are expected to be dropped (a season finale is not an easy place to introduce a new leading man), but also a remarkable feat of leak-plugging. Half the games industry should be taking notes.

Now let’s hope that RTD kept his nerve when he was writing…