LONDON -- Keira Knightley has come of age.

The 22-year-old star, who played a soccer-loving teenager in "Bend it Like Beckham" and a swashbuckling gentlewoman in the "Pirates of the Caribbean" movies, enters a world of adult sacrifice and betrayal in "Atonement."

In the film adaptation of Ian McEwan's novel, Knightley plays Cecilia Tallis, the bored and brittle daughter of a wealthy English family, who's frittering away a hot 1930s summer after finishing college. Her emerging romance with a housekeeper's son is destroyed by a lie told by her younger sister -- a fiction that transforms the lives of all three central characters.

The film, already being talked up as an Academy Awards contender, is likely to be a turning point for Knightley, co-star James McAvoy and director Joe Wright. For Wright, who also directed Knightley in 2005's "Pride and Prejudice," and McAvoy -- last seen as the Scottish doctor befriended by Ugandan dictator Idi Amin in "The Last King of Scotland" -- it could open doors in Hollywood.

For Knightley, already a big enough star to be a target for paparazzi and the tabloid press, it provided a chance to play a flawed adult character after a string of ingenues.

"I was interested by the fact that she really isn't very nice at the beginning," Knightley said. "She's a snob. I think she's probably a good person, but she's at a point in her life where she's sort of aware of her privilege but completely directionless. I was fascinated by this woman who is emotionally repressed and yet the emotions are bubbling away under the surface and just waiting to explode."

Knightley said the opening scenes -- shot with a hazy, nostalgic glow by Wright and cinematographer Seamus McGarvey -- "lull the audience into a false sense of security."

"You think it is going to be one sort of film, and then" -- in a scene that turns on the accidental sending of a sexually explicit letter -- "that preconceived notion of what the film will be is exploded."

McAvoy's subtle performance an Robbie Turner anchors the film, and dominates its quietly devastating central section, a recreation of the evacuation of British troops from Dunkirk in 1940.But while the 28-year-old McAvoy may be bound for stardom. Knightley is already there. She regularly graces magazine covers and has been pursued by paparazzi.

But Knightley, daughter of an actor and a writer, said she has not lost her love of the movies. She is currently back in period costume as the spirited and unconventional Georgiana, Duchess of Devonshire, an 18th century ancestor of Princess Diana, in "The Duchess," scheduled for release next year.