This is very much an episode of two halves. In many respects it carries forward all of series three's new strengths, but on the other it also has all the elements that have made the new series wobble so much since it came back.
It's basically well-acted, even though Tennant seems to have reached a plateau in terms of his improvements this series: he hovers over the edge of tolerability, for the most part behaving himself but still occasionally lapsing back into Mr Gumby-meets-Timmy Mallet, which turned me off him so much last series. Even now my lip still curls a bit at his “I'm kerayzee me, wahey!” nonsense. But for the most part, The Lazarus Experiment got off to an excellent start. With virtually no effects apart from the big Lazarus-machine set-piece, this episode relied on dialogue. And very good dialogue it was too: witty, mature and thought provoking, this was one of the few times where I felt that the writer was giving the audience some credit for once. Although, for the record, I don't care that it references The End Of The World. Or The Leisure Hive. Or This Is Spinal Tap. Or anything else.
Unfortunately this didn't last. I've been looking forward to seeing the Lazarus Monster in all its glory since first glimpsing it on the trailer at the end of The Runaway Bride. While it doesn't disappoint visually – it rivals the Reapers as the new series's best-looking original monster – the episode's real problem is that it has absolutely no idea what to do with it and too much of the story is just a perfunctory runaround up and down corridors. The moral implications of Lazarus's project are only half-touched upon at this point, but all the monster does is crawl round pipes, cast shadows and go “RAAAAR”. Admittedly it looks brilliant while it does it, but it stops being interesting very quickly. Martha's “intelligent modern woman” credentials take their biggest knock yet as it takes her a moment to realise that the roof is on top of the building, and the story chugs along hitting magic button after magic button like it's a massive game of Hungry Hungry Hippos.
The sonic screwdriver, the show's biggest plot stop-gap, is used to such a ridiculous extent here that it virtually undoes every previous episode's good work all by itself: it really is the most shameless substitute for telling a story properly, and Martha's question of “what's that thing gonna do?” really is absurd in context. Anything, that's what, including killing the drama stone dead. And that resolution – drop the word “hypersonic” into the technobabble somewhere so you can kill the monster with a church organ into which is plugged...guess. The Doctor's perfunctory speech about how humans have to face death doesn't have an original idea in it, and while the episode does interesting things with Martha's mother the same sledgehammer-subtlety comes in at the end and spoils things. Yet again.
As for this big final trap: I think it's good that they haven't foreshadowed things too much and I'm glad they've kept it vague enough to defy the ludicrous amount of speculation that's probably coming anyway. The marketing department's misdirection was appreciated too, flagging up Lady Thaw's character who turns out to be the first one dead. One question though: why try to practically deify the Doctor in that portentous monologue on the final trailer, when every second he's on screen Tennant makes the Doctor all the less mysterious and more like your annoying friend who keeps telling everyone how mad they are?
The verdict then: a roughly enjoyable episode, but when I think about what it could have been – and almost was at times – then it's a disappointment.