It's having a funny old run, this sixth series of Doctor Who. We've gone from a distinct 'meh' for the first three episodes straight to a 'Blimey, that was ace', for the fourth.
And then along comes episode five and it's another belter. 'Praise be', as the late Dame Thora Hird would have said.
You can never accuse the boffins behind Doctor Who of lacking in ambition. True, they sometimes over-reach themselves and don't quite manage to pull it off (See The Impossible Astronaut, Day of the Moon, Vampires in Venice. Remember Vampires in Venice? Exactly.) but what other Saturday evening family programme would ask you searching questions about the nature of life, creation and what it means to be human while running down corridors inside a Welsh castle? (Well, all right, probably something on BBC2 presented by Professor Brian Cox or someone dead clever like that. But can you imagine Prof Brian running down corridors? No, me neither. And he certainly wouldn't finish an edition of Wonders of the Universe with a melted face like the Doctor had in The Rebel Flesh. And even if he did he'd still spend ten minutes telling you how brilliant it was.)
So, yes, here we are in a futuristic factory where perfect clones - otherwise known as dopplegangers or 'gangers' - are created to carry out the tasks that are too dangerous for people to perform. But the factory has been hit by a solar flare and now the gangers are waking up and, because they share the same memories as their creators, they think they are real.
As I say, for a kids' TV show - and let's not kid ourselves, that's all Doctor Who is - the ambition really is quite amazing. At the heart of the plot is a simple question: If you have a clone and it's the same as you in every way, why is it not 'you'?
"I am Jennifer Lucas," said one of the gangers to Rory. "I remember everything that happened to her in her entire life. Every birthday, every childhood illness. I feel everything she's ever felt and more. I'm not a monster. I'm me."
The Rebel Flesh is asking exactly the same questions as Frankenstein, but it's pitching them at 6.45pm on a Saturday night, right before So You Think You Can Dance (a programme which, it's fair to say, is not exactly known for making the viewer think - unless it's about reaching for the off switch or heading to the pub).
And it was scary, too. If you read the grown-up newspapers there's been quite a bit of debate about whether or not Doctor Who is now too scary for children. Personally, I think scariness is the the point. It should be scary, kids should be hiding behind the sofa. After all, there's much worse horrors out there than melty faces and gangers sticking to the ceiling, and this episode certainly didn't hold back the shocks - particularly when it came to that ending.
"It's frightening. Unexpected. Frankly a total and utter splattering mess on the carpet. But I am certain, one hundred per cent certain, that we can work this out. Trust me. I'm the Doctor."
Can't wait, frankly.