Flying the flag for struggling humanity is the Grange research centre and its charismatic head of research Karl Baring – a man who suddenly goes missing in an incident which leaves his sister Katy in a catatonic shock.

Blithely breaching all the Grange's security, the Doctor strolls in and makes the mystery his own, rapidly realising that there is a troll at the centre of it all.

The clue was in poor Katy's words, something along the lines of not going under the bridge. After all, what else are you likely to find under a bridge other than a troll. And this one's a particularly nasty one.

The scientists are set on making England a green and pleasant land once again. The troll is a major stumbling block – especially when it emerges that Karl's identity and the troll's identity are worryingly blurred.

Not to worrying, though, for the Doctor who is at his charismatic best, willingly confronting danger, infuriatingly cheerful and somehow invariably above it all – a great piece of characterisation wonderfully captured by Tennant as one who should know.

Touching on all sorts of environmental themes, it mixes sci-fi, fairy tale and the future of our planet in a pleasing mix.

Phil Hewitt