Based on the 2002 Hong Kong thriller Infernal Affairs, Martin Scorsese's latest tale of betrayal and deception in the underworld melds stellar performances with earthy dialogue, punctuated with explosions of violence. The Departed bears all of the veteran film-maker's hallmarks, but while this thriller may finally earn Scorsese the Academy Award that has eluded him all these years, he won't truly deserve it.

From a technical perspective, the film offers little that we haven't seen before from the master director and there's a noticeable lack of brio in the action set pieces. The film succeeds thanks to the magnificent efforts of his regular collaborators, not least editor Thelma Schoonmaker, whose mastery of pacing makes light work of the excessive 151-minute running time.

Leonardo DiCaprio continues to mature as an actor, revelling in the ambiguities of his character, and there is strong work too from Matt Damon as his nemesis: two snakes in the grass who must bite, or be bitten. Screenwriter William Monahan transplants the action to South Boston, where organised crime rings are locked in a bitter battle for supremacy with the Massachusetts State Police. The police's chief target is mob boss Frank Costello (Jack Nicholson) - put him behind bars and his entire empire will crumble to its foundations.

So Captain Queenan (Martin Sheen) and hard-nosed Sergeant Dignam (Mark Wahlberg) decide to destroy Costello by planting a mole in his close-knit operation. They approach young recruit Billy Costigan (DiCaprio) to go deep undercover, including a brief spell behind bars to forge a reputation as a good cop-turned-bad. Costello appears to fall for the ruse and he slowly welcomes Billy into his operation, learning the ropes from his associate Mr French (Ray Winstone).

However, the police are unaware that one of Costello's underlings, Colin Sullivan (Damon), has successfully wormed his way into the upper echelons of the force. Working alongside Captain Ellerby (Alec Baldwin) on the Special Investigations Unit, Sullivan is party to vital information about the secret operations, and he drip-feeds the information back to his boss. Costello and the police soon realise they have double agents within their organisations, and both sides put plans into action to smoke out the traitors.

The Departed plays to its strengths, including a terrific ensemble cast and impeccable production design, editing and a thrilling orchestral score from Howard Shore. DiCaprio and Damon command the screen and in his latest, larger-than-life supporting turn, Nicholson merrily steals every scene, blessed with all of screenwriter Monahan's best lines and an unforgettable revelation' in an adult cinema. He delights in his character's earthy dialogue. "How's your mother?" Costello asks an acquaintance. "She's on her way out," comes the sombre reply. "We all are," responds Costello, "Act accordingly!"

Meryl Streep seems destined for her 14th Oscar nomination for her tour-de-force portrayal of a domineering magazine editor in The Devil Wears Prada, the smartly tailored film version of Lauren Weisberger's international bestseller. Gliding through every frame in swathes of Galliano, Valentino and, of course, Prada, the veteran actress is utterly breathtaking as the sadistic reigning queen of fashion, who is armed to her polished teeth with a dizzying array of knockout one-liners.

As despicable as her character may be, Streep expertly reveals the chinks in her villainess's designer label armour, showing glimmers of humanity and vulnerability beneath the impeccably coiffed facade. We come to love this silver-haired bully - the way she terrorises her staff, her refusal to tolerate fools - at the expense of the heroine of the piece, Andy Sachs (Anne Hathaway), a graduate from Northwestern University who lands a plum job as second assistant to the fearsome Miranda Priestly (Streep), esteemed editor of Runway magazine.

Andy hopes the position will be a stepping-stone to serious political journalism and is unprepared for the immense challenges that lie ahead, catering to her tyrannical boss' every whim. Beleaguered first assistant Emily (Emily Blunt) soon puts Andy in the frame about Miranda: "She is not happy unless everyone around her is panicked, nauseous or suicidal."

Andy quickly understands Emily's grim forecast. Within hours of arriving, she is black and blue from Miranda's stinging barbs and when Andy dares to ask for clarification, the editor cuts her stone dead: "Please bore someone else with your questions." Delivering the killer blow to her underling, Miranda cruelly reveals why she hired Andy, a devotee of thrift store chic, rather than another stick-thin beauty with a passion for fashion. "I said, Take a chance: hire the smart, fat girl."' Determined to make Miranda choke on her acidic words, Andy seeks guidance from camp art director Nigel (Stanley Tucci), who immediately summarises the problem: "You are in desperate need of Chanel." Using Miranda as her template, Andy undergoes a drastic personality change, which alienates her from boyfriend Nate (Adrian Grenier) and family. Decked in designer labels and looking just like the other sycophants that surround Miranda, Andy realises that she has become part of the crowd. Unless she acts fast, her individuality will be lost forever.

Hathaway fades to grey in Streep's considerable shadow. Her fashion illiterate twenty-something is too wet and simpering to merit our affections.

If anything, we delight in her suffering under Miranda.

Supporting performances are much more than window dressing. Tucci is a hoot and British actress Blunt is utterly adorable as the stressed out gopher with a unique approach to weight loss.

"I'm on this new diet," she confides. "I don't eat anything and then when I feel like I'm about to faint, I eat a cube of cheese."