A Mother's Son, ITV1

We have been here before: The Killing wasn’t the first crime drama to open with a damsel in distress. This time it’s a schoolgirl who is being chased across the sand dunes at night. She has been stabbed. She falls – conveniently backwards – to the ground. The pursuer is reflected in the dying pupil’s dilated pupil. “I’m sorry,” whispers the girl. Why?

The first part of A Mother's Son throws up many more intriguing questions. The second one is why the script editor allowed Chris Lang to repeatedly commit the cardinal sin of letting his characters tell each other what they already know. “My daughter is 17 years old, Ben,” says Rosie to her partner. They have only been together for seven months and three days. The lovey-dovey duo, each with a son and daughter from a previous marriage, share a splendid Gothic pile in Suffolk.

After this shaky start the show soon hots up. Why does Jamie, Rosie’s teenage son (pictured right), root through her bedside drawer? Why has he uncharacteristically put his school uniform in the washing machine? Who is spying on Ben’s nubile daughter when – shades of Psycho (another tale of a disturbed mother’s boy) – she takes a shower? Why does Jamie insist that he has lost his trainers? He doesn’t know that Rosie has found them under his bed – splattered with blood. Ben supplies one answer: he’s “an inveterate liar”.

As news of the schoolgirl’s murder spreads through the seaside town, suspicion and rumour ripple through the modern family like wind through rye-grass – images of which punctuate the narrative. The step-brothers fall out. Rosie, who works behind the counter of an upmarket deli, can’t help putting two and two together but refuses to make four. Instead of confronting Jamie she confides in her ex-husband David (Paul McGann) who agrees to shadow his son. Cue shots of Jamie smoking dope with a truly dodgy character. Rosie, meanwhile, pays a greedy nerd to discover the password to Jamie’s laptop. His recent history includes such words as porn, gangbang and rape.

If you go looking for trouble you’re bound to find it. The strength of Lang’s script lies in the way it slowly accumulates detail and spawns red herrings. Jamie is by no means the only one who could have killed the girl. Ed Bazalgette’s production is full of telling images. When a distracted Rosie stares through the window of the deli she catches a delivery guy casually checking out some passing schoolgirls.

The excellent cast creates a palpable atmosphere of bitter resentment and bitten tongues. Hermione Norris (pictured left) as Rosie is the ultimate helicopter parent, hovering round her kids until adoration gives way to exasperation. Martin Clunes, all bumbling, puzzled devotion, turns Ben into a Dickensian dupe. Then there’s Nicola Walker as the investigating detective, and the always brilliant Annabel Apsion as the bereaved mother: both work wonders with their minor roles. As for Jamie, mop-headed Alexander Arnold, mixing vulnerability with menace, mopes around to great effect.

Did he or didn’t he knife his schoolmate? Will Rosie fall back into David’s arms or won’t she? Only one thing is certain: the second part tomorrow will end in tears. Let’s hope the answers don't leave us feeling sorry too.