There’s something very disturbing about seeing Anneke Wills playing “Michelle Leuppi from Zurich” quite differently from Polly: a cold, dismissive lack of interest in other people, curt to the Doctor and Samantha Briggs. No wonder the Doctor quickly surmises this isn’t Polly brainwashed – there’s nothing of her personality in Michelle Then there’s the double punch of Ben discovering the real Polly’s body sealed, like a waxwork, inside a crate. It’s a shame this is more or less her final episode, but at least she goes out with an acting challenge. Michael Craze gets less to do – but gets an almost equally brilliant moment of horror when he’s shot as he talks to the Doctor on a video screen, and the Doctor can do nothing to help.

The other really nasty part of the episode is the Chameleons themselves – it’s hard to tell from the telesnaps, but they seem to be revealed in a fairly casual way, with the sick one from last week lying on a bed in the hospital looking like the crime scene photos of Mary Jane Kelly – violently faceless, like a carved piece of beef. The plan to transform these creatures into copies of British citizens is a Doctor Who twist on any number of Sixties telefantasy series where Eastern Bloc agents are turned into doppelgangers (e.g. The Avengers: The Man With Two Shadows).

It’s not all grim though: there’s a lot of humour, like the Doctor and Jamie hiding from the police behind copies of The Times (which Jamie, unable to read, holds upside down). And even the exposition scenes are kept quirky and interesting. Samantha Briggs, investigating under her own steam, is such a brashly modish character with her massive hat and Mersey accent and crush on Jamie. Her no-nonsense persistence panics the Chameleons. And the info-dump between the Doctor, Jamie and Ben takes place in a passport photo booth, which gives the regulars an opportunity to muck about:

With the Doctor finding clues in newspapers, the police, in the form of Detective Inspector Crossland, and the sister of one of Chameleon Tours’ young victims all closing in, this definitely has the feel of a TV thriller – of a sort this show hadn’t really attempted before. This fits into a respectable strand of adventures that involve dropping the Doctor into a different genre, and I’m very glad they tried it.

Next episode: The Faceless Ones – Episode 3