Last Night's TV: Criminal Justice, BBC1

Apparently, some lawyers have already complained about the way they are portrayed in Criminal Justice, and going by the first episode, it's certainly not an advert for the profession.

Banged up in a police cell for a murder that he almost certainly didn't commit, young Ben Coulter (Ben Whishaw) was jerked out of self-pity by the arrival of the duty solicitor, who announced himself with the words, "My name is Stone. Shut up. Good boy."

Stone is, to judge by appearances, every innocent man's nightmare: unshaven, shabbily dressed, shoulders sprinkled with scurf, toes – poking out of sandals of dispiritingly orthopaedic appearance – wrinkled raw with eczema. And if going by appearances strikes you as shallow, try going by what he says.

His idea of bonding with a client is to point out that Ben's asthma and his skin condition have similar pathologies, and when Ben said he wanted to tell him the truth, Stone seemed affronted: "You really, really don't. I don't wanna be stuck with the truth."

Mind you, you can see his point. So far, the glaring flaw in Peter Moffat's script is the almost perfect storm of circumstances gusting Ben towards prison.

Having borrowed his father's black cab when his own car wouldn't start, Ben then found he didn't know how to switch off the yellow "For hire" light at the front – d'oh! Stopping to answer his mobile – the mate he was supposed to be meeting cancelled on him – he found that a pretty young woman had assumed he was pulling over for her, and had already jumped into the back seat – ouch!

One thing led to another and they ended up back at her house, where – as a preliminary to sex – she fed him tequila and cajoled him into playing the game where you follow every shot by stabbing the table between your fingers with a sharp knife. He missed the table and got her hand, leaving both him and the knife he'd been handling covered with her blood.

Perhaps surprisingly, they went ahead with the sex, so that when the young lady was subsequently murdered – stabbed in the heart while, so far as we can tell, Ben was comatose over another tequila in the kitchen – the police were presented with as open and shut a case as they can ever have come across. And Ben didn't do himself any favours by speeding off in his dad's black cab, then zipping back to retrieve his jacket and wipe away a few bloodstains. Short of stopping at an all-night tattoo parlour to have "It's a fair cop, I done it all right" inscribed on his forehead, it's hard to see how he could make himself look any more guilty, and the assumption by the fuzz that he done it does strike you as, well, not altogether unreasonable.

But while all this strained credulity somewhat, it was gripping – Whishaw and Ruth Negga, as his brief inamorata, both being credibly attractive and in the grip of attraction – and once Ben had fallen into the clutches of the law, both tempo and plausibility were ratcheted up several degrees. Moffat was previously responsible for the classy, but lamentably short-lived legal drama North Square (and Phil Davis's star turn as Peter McLeish, the feral, scruple-free lawyer's clerk was at least in part the template for Con O'Neill's Stone), but his talent for handling dialogue has improved considerably since then.

A key scene in this opening episode came during Ben's interview with Box (the great Bill Paterson), the detective superintendent in charge of the investigation, labelled by Stone "a talented oppressor... a subtle beast". (Box's opinion of Stone was somewhat lower: "A slappers and dippers man.")

Puzzled that Box hadn't got around to charging Ben, Stone taunted him: "He doesn't feel right, does he? You're very proud of your gut and your nose. But neither of them are fancying my boy for this, are they, Harry Box?" It's the way the christian name was tacked on at the end, an additional taunt, letting him know how well he knows him, how transparent his weaknesses are.

And of course Box likes Ben. What Whishaw brings to the part is an appallingly vulnerability – rakishly thin, his eyes dark and wide under a childish mop of hair. Even the policewoman who brought him in, and who knew what he had left behind in that house, couldn't help turning a little motherly.

If I had to pick out a bit of acting, though, it would be the moment when Ben asked Box, as a swab was about to be taken from the inside of his penis, whether it would hurt. Paterson's "Yes" managed to pack in both compassion and a sense of satisfaction that the lad was getting what he deserved. Possibly the lawyers aren't coming out too well, but everybody else – including, for a change, the police themselves – is doing all right.

If there is a double meaning wedged into that title – a hint that our system of justice is itself criminal – it's not born out by the action so far. The only ominous note was the presence, in the van taking Ben off to prison at the end, of a cartoonishly psychopathic prisoner who's evidently taken a dislike to him: melodrama looms on the horizon. But until it's definitely here, this looks like a series worth sticking with.

Brian Viner is away