England 1935. It's the hottest day of the year and the cut-glass Tallis family are convening at their Gothic country pile.

Thirteen-year-old Briony (Saoirse Ronan) is keen to dragoon her reluctant cousins into performing her first self-written play.

Her elder sister Cecilia (Knightley) drifts through the shimmering heat-haze lethargically awaiting the arrival of the family's aristocratic guests.

However, the air of dull dread as war looms is shattered when the wilful Briony spots her half-naked sister and the housekeeper's son Robbie (McAvoy) by the fountain.

Misreading the situation from afar, Briony's wild imagination sets in train a series of catastrophic events that will see Robbie accused of a terrible crime he did not commit.

To reveal much more of the plot would seriously damage the enjoyment of a fledgling romance struck down by a moment's act of betrayal.

Knightley and McAvoy, despite occasionally slipping into Brief Encounter mode, thoroughly convince with taut, luminous performances as the lovers cursed just as passion blossoms.

Director Joe Wright and screenwriter Christopher Hampton have skilfully teased out the themes of guilt and remorse from Ian McEwan's novel, cleverly wrongfooting the viewer with devices you'd expect to see in a thriller.

It's a sophisticated, grown-up drama that also manages to touch on the horrors of war (a seven minute tracking shot of the Dunkirk evacuation is stunning) and the supercilious duplicity of (some of) the ruling classes.

But most of all it's a rivetting love story and a profoundly moving meditation on what might have been.

Go see it. You'll be sorry if you don't.

Tim Evans