It might occur to you after watching “The Ghost Writer” how little the mystery really needs to matter in a mystery. The film’s would-be stumper isn’t much of one. The solution doesn’t impress. Yet you move slowly and unstoppably into its silent unease. “The Ghost Writer” is, predominantly, a film of mood and execution. As such, Roman Polanski’s new film is one of my easiest top-graders and something hovering around a masterpiece. Reminiscent of “Chinatown,” Polanski turns the summer elite playground of (a fake) Cape Cod into a rainy pit of deception and self-deception, of femme fatales and dark humor, of paranoia and lust and dread. Homes become traps. Beaches become prisons of muddy sand. “I feel like the wife of Napoleon on St. Helena,” says one character, bluntly and bitterly.

A writer known only as “The Ghost” (Ewan McGregor) is not on vacation. He is there working. He will make a huge score to polish a British Prime Minister’s autobiography and walk away anonymously. The timing is awful. There’s a love triangle. A frustrated wife. And a political crisis over acts of torture by the Minister’s government. To top that, the first ghost writer took a mysterious and permanent plunge into the Big Drink. A man smart enough to know better, the writer starts looking into the “suicide.”

The sublime dialogue of the script––written by Polanski and the novelist Robert Harris, on whose 2007 novel the film is based––is a thing of sophistication. A mixture of English dry humor and American hardboiled lingo is delivered with gentile bite by the tragically overintelligent. When Ewan MacGregor suggests to a smoking and smoky Kim Catrall “Maybe you’d like to search me,” it’s the sort of line that you would imagine roll out of Humphrey Bogart’s mouth. Every word seems to carry a secret meaning – sexual or perilous, perhaps warnings of each.

As the film’s isolated hero and patsy, McGregor is silently intense, shrewd but vulnerable. He is matched and probably outdone by Olivia Williams as the Prime Minister’s Lady MacBeth, a slick, secretive steel matron losing her grip. Along with a memorable turn in “An Education,” it’s been a strong year for an actress who has been missing too long, As the neocon devil, Tom Wilkinson makes every polished word feel like it hides a dagger.

After Polanski’s recent arrest, it occurred to me that the key to “Chinatown”’s ending is the failure of Jack Nicholson’s imagination. Incest is outside the realm of Jake Gittes’ — and the audience’s – range of possibility. With “The Ghost Writer,” all the clues stare us in the face. Yet the writer only slowly comes to see past the comfort of the official stories. A mystery is supposed to restore our sense of order in the world, but “The Ghost Writer” subversively denies us what we most want: our sense of justice.