An idyllic day in the country turns bleakly sour when a desperate bid to harness a runaway hot-air balloon with a child on board ends in death.

Four men attempt to hold the wind-tossed canopy but one tumbles fatally to terra firma after the others let go of the ropes as the balloon rises.

That night would-be rescuer Joe (Craig) and his girlfriend Rachel (Morton) recount over dinner with friends how he was helpless to avert the tragedy.

What makes his dilemma - if he had held on would the man have survived? - particularly hard to take is that the balloon went on to land safely.

It's a case of "if I'd done that" or "we should have done this" but what he doesn't realise is that his fleeting acquiantance with another helper is where the real nightmare begins.

Jed (Ifans) calls Joe up and requests a meeting to discuss the trauma he's feeling after also witnessing the balloon victim plummet to his death.

Lank-haired and dishevelled, it's a bit like being bothered by a distracted Paul Weller - but Joe reluctantly agrees. After all, they are bonded of sorts by common experience.

However, what was once a minor irritant - "I'm sure it's harmless; I feel a bit sorry for him" - becomes a malevolent threat as Jed stalks his every move.

Reasoning is no use and insults worse while Jed's obsession and Joe's delicate state of mind mean his relationship with Rachel is under threat... and worse.

Terrifically natural performances - particularly from the ever-impressive Craig and Morton - are serviced by a richly shaded script.

The sublimely executed opening where the hapless victim is ethereally cast adrift into thin air and certain death, hooks the viewer and never lets go.

Questions are also raised about love - obsessive, nurturing, destructive and, of course, enduring - but the main thrust of the narrative concerns obsession.

Jed's is of a type - de Clerambault's syndrome - where amorous attraction is fuelled by everyday acts - opening and closing the curtains - and is interpreted as a come on.

There's a fairly redundant sub-plot concerning an apparent affair the victim was involved in and you do wonder why Joe doesn't call the police.

However, overall it's a stunning piece of cinema; intelligent, disturbing and all-too-believable.

Tim Evans