Black Work, ITV

Drama is all about secrets revealed, discoveries unfurled. Black Work was straight into that territory from the first scene. A man and a woman sat in a car, taking the solace from each other that they couldn’t find at home. As ever in such a scenario, you promptly wondered if or when they’d be caught in the act.

She sounded keener on rescuing her marriage to a mostly absent husband. But the next time he went out to work he failed to come back. The sight of a WPC on the doorstep told her everything. Actually not quite everything. It turned on how her husband was shot dead in an abandoned warehouse by a gang of gun-runners, and that he’d been working undercover for three years to reel them in.

The ingredients are all there for a taut thriller in which no one seems to be telling anyone else the truth

Matt Charman’s script riffs on the murky morality of police infiltrating the criminal underworld using a false identity. It deftly exploits the idea that nobody is necessarily who they say they are. The first time we met them, it wasn’t mentioned that all of the love triangle – Jo Gillespie (Smith), her husband Ryan (Kenny Doughty), and her bit on the side Jack Clark (Matthew McNulty) – were coppers from the same northern nick. The aura of secrecy deepened further when Jo was asked by Ryan’s boss DCS Hepburn (Douglas Henshall) to keep a lid on her heartbreaking loss for 24 hours, not even to tell the dead man’s children. And then Jo discovered that Ryan had also been working undercover closer to home, taping her furtive car conversations. A mere constable, she soon went rogue to solve the mystery of Ryan’s death herself, and unpick his alternative identity.

The ingredients are all there for a taut thriller in which no one seems to be telling anyone else the truth. With two more episodes to come, some of the plotting so far feels a bit gauche: Gillespie’s little daughter found the adultery tapes in the airing cupboard and blithely gave them a different sort of airing, while Ryan’s mother accidentally blew the story of her son’s undercover work by phoning up the local press. And the script was weighed down with heavy portents.

Smith as ever has the ability to inject compulsion and depth where it might otherwise be missing. It’s something to do with the face, which is little and round and serviceable, and here framed by a short purposeful crop. It's the face of an ordinary person with an extraordinary access to charisma. Emotions which wash across it, from wounded to war-like, never look like acting, or in the service of a story; they just are.

Of her co-stars, Henshall was creepily sincere as he spouted empty reassurances, while Geraldine James was stately as a latter-day Jane Tennison, running the department while keeping herself trim on jogs in lycra. But it’s a measure of quite how good a performer Smith is on the small screen (as much as on the stage) that everyone else here looked as if they were on half rations. She alone encourages a curiosity to know what happens next.