"These aren't the old days," cautions one desk sergeant in Prime Suspect 1973, a new police procedural that is set, resolutely, in the old days.

"Mum, it's 1973, it's not a big deal," says a young Jane Tennison (Stefanie Martini), in a moment of chronological defence, real purpose being to remind us that it really is a big deal that it's 1973. This is because we are watching a prequel to Prime Suspect, the long-running drama starring Helen Mirren as one of London's first detective chief inspectors (DCI), and later detective superintendent (DS), set back when she was just a fledgling woman police constable (WPC).

Based on Lynda La Plante's recent novel Tennison, Prime Suspect 1973 is suffused with period detail, flagrant sexism and clues to the redoubtable detective Tennison will become; in other words, the making of a BFD.

Two episodes in, though, the makers have to contend with the fact that, while Tennison is the protagonist, she is still a marginal figure: there are places she simply can’t go. The station is an unthinking boy’s club where WPCs are routinely asked to answer the phones, make tea, and clean up interrogation rooms, which they do with quiet acquiescence.

Like La Plante’s original, which ran off and on for seven series between 1991 and 2006, the new show endeavours to layer the procedural with the political. Tennison’s involvement in the murder investigation of a young woman is at first almost accidental, selected as a sympathetic face to inform the victim’s parents, but she pushes discreetly and firmly to remain on the task.

Deliberately excluded

In a shrewd sequence, as rich in detail as any of the programme’s painstaking period props and exteriors, Tennison enters an office thick with the smoke and laughter of men, and time seems to slow with a joke that deliberately excludes her. Rumours of a romantic encounter with her superior (Sam Reid) are circling, well founded as it happens, but only Tennison stands to be diminished by them.

Here the show is goading its audience: Tennison, who looks so much like Twiggy it seems anachronistic that nobody mentions it, resents the suggestion that she joined the force to bag a husband (“Like I’m some cat up a tree waiting for a man to save me,” says the dry Martini). But she and Reid have such clear sexual tension you’d need to belong to a HR department from the future to advise against them going further.

In one of the show’s smart, if typically serendipitous moves, Tennison makes a breakthrough by deploying her sharp observations within her professional restrictions: treated like domestic help, she moves unobserved, with greater access, and notices a sinister connection at the home of a grieving family. “Good spot,” she is later commended.

Prime Suspect 1973 would like to see further, but it feels thinner than its 1990s forebear, spread out across six shorter episodes and presenting its protagonist as someone waiting to acquire a character. The future will be better, it promises Tennison, if not fairer. In 2017, it's still a big deal.