In advance, Ludwig sounded as if it was aimed squarely at the Inspector Morse market. Set among spires of impeccable dreaminess (in a cunning twist, those of Cambridge), it has a main character who solves crimes and cryptic crosswords with equal efficiency.

Once the series began, though, it was clear that its sights were set a little lower than that. Instead, the show seems content to take its place as the latest proof that plausibility is out of fashion in TV drama these days. (In my last column I reviewed Nightsleeper, which had no time for it at all.)

One reason this detective feels like the traditional fish out of water, for example, is that he’s not a police detective. When we first saw John Taylor (David Mitchell), he was passing the time in what was evidently his usual way: alone at home either setting or airily completing newspaper puzzles. (Ludwig is his crossword-compiling nom de plume.)

But then came a call from his brother’s wife Lucy (Anna Maxwell Martin) summoning him to Cambridge where she had a simple request: would he please pretend to be his identical twin James, go into the local nick where James was a senior cop and retrieve his........

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