Costume dramas belong on a Sunday night. My only real concern about Parade’s End was the somewhat perverse decision to broadcast it on a Friday.

Conversely, a good detective yarn finds its natural home at about 9pm on a midweek evening.

The Bletchley Circle is both costume drama and good detective yarn. Combining those elements has conjured a mystical alchemy that makes the show watchable on any day of the week.

The first thing you see is a caption telling you it’s 1943. They needn’t have bothered. You know it’s the 1940s because absolutely everything is brown. The story centres on four extraordinary women – each with a ‘superpower’ that when combined makes them a formidable team.

The show opens on them working together at Britain’s ultra-secret wartime code-cracking facility Bletchley Park. Anna Maxwell Martin, the reigning queen of modern period drama, is pattern recognition expert Susan. Rachael Stirling, dazzling in Tipping The Velvet and resembling her mother Diana Rigg more with every passing day, is the team’s mathematician Millie.

Julie Graham, who seems to have been in just about everything is information-gathering genius Jean. The youngest member of the team, gifted with a prodigious eidetic memory, is Lucy, played by Sophie Rundle. She’s about the least famous of the four principals but you may remember her from Julian Fellowes’s Titanic miniseries.

A lot of women found it difficult to settle back into their pre-war domestic rôles after 1945. The kind of fiercely clever woman who worked at Bletchley probably felt that more keenly than most. That frustration must have been sharpened by the need to keep their war-work secret for decades.

That’s where Maxwell Martin shines. Every inch of her disappointment at going from Enigma decrypts to knitting patterns is hewn on that inimitably expressive face. The silence that ensues when a perceptive policeman asks her directly about her war service is one of the greatest, most meaning-imbued silences in television history.

Most of the action takes place in 1952. There’s a caption telling you it’s 1952 but they needn’t have bothered because two young children ask if they can play outdoors and their mother says “yes, of course”.

Susan decides to put the crack wartime team back together to track down a serial killer. There’s a great deal of set-up in Episode 1, but it’s pretty engaging set-up that you won’t mind being part of. Episode 2 promises to be a distinctly faster-moving business.

Is it worth watching? You bet it is. It’s worth watching for the coats alone.

Anna Maxwell Martin seems to have a clause in all her contracts that there’s a lovely coat included as part of her costume. The Austerity/New Look affair she wears in The Bletchley Circle isn’t quite up the standard of the splendid manteaux she wore in South Riding or The Night Watch but it’s still pretty special.

The Bletchley Circle goes out at 9pm on Thursday evening but you could just as easily watch it on any day. Friday, for example. As long as you have the technology to time-shift Parade’s End to Sunday, where it belongs.