The Bletchley Circle, Series 2, ITV

For a drama as committed to the exploration of the changing role of women in post-war Britain, The Bletchley Circle isn’t above a little sleight of hand. The second series of the critically acclaimed whodunnit began with a flashback to 1943 and to Alice Merren (Hattie Morahan), a bright young codebreaker who quickly solves a puzzle that the menfolk have been bamboozled by for the past two days.

It’s people and the mistakes that they make that form the centrepiece of the first of the brace of two-part stories making up the second series of The Bletchley Circle. When the action shifts to 10 years later - and one year after the 2012 three-part serial was set - Alice is found covered in blood at the home of one of those menfolk, John Richards (an all-too-brief appearance from Paul McGann). She is ultimately found guilty of his murder and sentenced to hang, but refuses to say a word in her own defence.

That’s where her former colleagues, led this time by boss Jean (Julie Graham), come in. The first part of the mystery is hardly the Engima Code: the relationship of Alice (Morahan pictured right) and Richards is established before the credits even roll, and where wartime star-crossed lovers are concerned there’s only ever one explanation for why a woman mistakenly accused of murder will keep her own counsel. Although the codebreakers-turned-sleuths take some time to get there - and only then thanks to the deduction powers of the reluctant Susan (Anna Maxwell Martin) - they do so in four fine performances, complete with just enough of a flavour of the characters’ back stories to keep new viewers hooked.

Of the four, which also include maths genius turned Scotland Yard employee Lucy (Sophie Rundle) and Rachael Stirling’s glamorous translator Millie, it is Maxwell Martin’s character that is the most intriguing. Susan’s initial reluctance to help Jean save Alice is never explained as the result of her experiences in the previous series, in which she acted as the lynchpin of the group, but her quick wit and inability to stay away tell their own story. However a subplot, in which she wrestles with the decision whether to move her family and follow her husband abroad on a Foreign Office posting, is perhaps given a little too much weight by the lack of exposition.

The episode got a little silly once Alice was ultimately found guilty of murder and able to open up to her friends about the identity of the mysterious young woman in the green or blue coat that Richards sent flowers to, not least because of the group’s utter lack of precautions even as it becomes more obvious that some sort of conspiracy is afoot. The slow-burning nature of the drama meant that the nature of that conspiracy, and the stakes, only became obvious in the closing minutes - setting things up nicely for next week’s concluding part.