Exile, BBC One

In a week unfeasibly packed with new drama across the BBC and ITV, the three-part Exile may prove to be the one that lingers longest. It was a thriller and a detective story, but what gave it its formidable grip was the way the central mystery was intricately entwined with the painful personal story of  Tom Ronstadt (John Simm) and his father Sam (Jim Broadbent).

Simm's character was a burnt-out journalist from the fictional London-based Ransom magazine. Until he got the sack, he had specialised in high-octane sleaze, his dirt-digging zeal cranked to a frenzy by drink and drugs. His father had been a journalist too, but had earned respect as a diligent investigative reporter in his native Lancashire. Tom had quit the family home and headed south years earlier, after his father caught him snooping in a secret file in his office and administered a ferocious beating which had severed their relationship.

Exile was the story of Tom's return to his hometown roots and his quest to find the meaning of that ominous file, which led to the fateful unpicking of his previously unknown family history. A dark and bitter past it was too, centring on the morally and professionally corrupt Donald Metzler (Timothy West), once a patient-abusing doctor at the Greenlake mental hospital, now a local entrepreneur and leader of the council. Long story short, Tom was the offspring of a psychiatric patient raped by the depraved Ricky Tulse, Metzler's subservient partner in moral squalor. Sam's urge to publish the truth was sabotaged by what the revelations would do to his own family.

Tom's struggle to get the real story was complicated by the fact that his father was now an Alzheimer's sufferer, only capable of offering the occasional random shaft of clarity from the accelerating disintegration of his mind. The idea of the disease as a sort of metaphor for the unbearable stresses of the past wasn't laboured by writer Danny Brocklehurst, but the relationship between father and son was movingly drawn. Tom's disbelief on returning home to discover the truth of his father's condition was brilliantly played by Simm, who at first couldn't grasp the delusions and loss of identity that were tearing down Sam's personality.

"You couldn't get something quick like heart disease or cancer," he raged at one point, boiling with the frustration of trying to talk rationally to a mind only functioning in fragments, none of them joined together. "You had to make us all suffer." But trying to piece together what little was left was the start of his growing dismay at how far apart they'd grown and how much time had been lost.

Broadbent (who's a patron of the Dementia UK charity) can rarely have been better, jumping seamlessly from benign passivity to hysteria, anger, terror or sometimes surreal humour as he traced the irrational patterns of Sam's behaviour. A moment of tenderness with his daughter Nancy (Olivia Colman, also excellent) was shattered abruptly when Sam suddenly shouted, "You're not Edith!" - his dead wife - and slapped Nancy across the face. Then he lapsed back into a tranquil calm as though nothing had happened, since, in his perception, it hadn't.

Despite the bleakness of the story, Brocklehurst had managed to insinuate mood-lifting touches of humour and deadpan observation. There was a superb scene where Tom hid ludicrously behind a supermarket stacking trolley to avoid his old schoolfriend Mike (Shaun Dooley, pictured above with Simm), whose wife Mandy (Claire Goose, above) he just happened to have slept with. As the relationship between Tom and Mandy developed, they fell into a bantering rapport that felt touchingly natural, as the shell Tom had erected around himself decades earlier began to crumble.

Brocklehurst had saved a bit of extra juice for Tom's final scenes with Sam, now in a care home. "You were this man... and you were this man," said Tom, showing Sam a series of photographs from their family past. "But you're not him any more. You're already gone." It precisely caught the terrible strangeness and sadness of dementia.

• Watch Exile on BBC iPlayer
• Find Jim Broadbent on Amazon