Coming Down the Mountain BBC1, 9pm

RARE indeed is the Sunday-night telly drama that will carry you along with it on a journey from the wintry depths of harrowing despair to sun-dappled peaks of joyous hopefulness: Coming Down the Mountain was such a drama.

An affecting and unforgettable portrait of two adolescent brothers in extremis, it sprang from the pen of novelist Mark Haddon, author of the justly-lauded serio-comic gem The Curious Incident of the Dog in the Night-Time. The narrator of Haddon's book is an autistic teenager; Coming Down the Mountain's central interior-monologuist was an alienated 17-year-old, David, who felt his life unfairly blighted by the heavy weight of familial attention trained on his Downs Syndrome sibling, Ben.

In addition, David greatly resented being Ben's day-to-day minder, ensuring his safe transport to and from an inner-London secondary school. His resentment took the form of describing his brother as a sub-human cartoon cross between a vegetable an an outer-space alien, "a big potato with eye-tentacles".

So far, so cruel in a routine knockabout teenage manner. But it wasn't long before you recalled the first dispassionate words voiced by David when Coming Down the Mountain began: "Last year, I decided to kill my brother."

David's route to murder began, as so many murders seem to do, when he invested all his love in the wrong woman - in this instance Gail the demi-Goth, an inconstant 16-year-old temptress with a mild death fixation and a piercing in an exciting forbidden place.

Given his angst-ridden teenage status as a flawed and uncertain adult-in-waiting, the self-contained David was completely swept away by his first intoxicating experience of sharing all of himself with another in a grown-up amour.

Sadly, though, Gail's love was out of mind as soon as David was out of sight, abruptly re-located from urban London grooviness to the benighted backwaters of rural Derbyshire by his parents (so Ben could enjoy a more sympathetic educational regime). A yawning pit of despair opened up at poor broken-hearted David's feet, and he couldn't resist diving in, head over heels.

Coming Down the Mountain's evocation of all the agonies attendant upon being dumped for the first time can rarely have been bettered on the screen, whether large or small: we shared every tortured second of poor young David's bafflement, anger, helplessness, black depression and sense of total humiliation.

We felt his growing impulse to self-negation and self-harm. We crouched next to David in his bathroom, whipped and beaten by the callous complexities of the human heart, as he sliced a blade across his arm in a listless suicide bid.

We were also there to understand how a deviant seed could start to germinate inside a lost, lonely kid's head or, as David put it: "Then it dawned on me - I'm hurting the wrong person'." - ominous words that signalled David's absorption into hellish inhumanity.

What happened next was the stuff of dread: David spiriting his brother away to a Welsh campsite and thereafter up a precipitous Welsh mountain side, driving him along with monstrous cruelty to its summit.

What then happened - a prolonged sequence of genuine catharsis, leading to salvation and reconciliation between the two brothers - was the work of a master dramatist (although surely Mark Haddon over-egged his pudding a little in creating two parents so completely oblivious to one son's personal growth and the other's personal suffering).

Hats off, too, to Nicholas Hoult for embodying David's pained gawkiness, and to Tommy Jessop in his role as Ben, the wise, perennially frank innocent.

Coming Down the Mountain: run up several flags to salute its fierce and sustained brilliance.