It’s hard not to picture Polanski under house arrest in Gstaad editing his diverting new thriller, in which a former British prime minister dodges extradition while having his memoirs rewritten. Then again, when your life is like a mash-up of the History Channel’s entire catalog of shock programming, autobiography will probably influence your fictions, and Polanski seems inspired as he maintains implausible momentum with a cloudy premise. Ewan McGregor diffidently plays the so-called ghost to exiled politician Adam Lang (Pierce Brosnan), living in cushioned seclusion off the gray New England coast. The writer is hacking through the bombast left by his (dead) predecessor when Lang’s past war-on-terrorism overstepping raises Blair-style static. Saved by often delightfully bitchy British dialogue, the movie sees McGregor’s (arbitrarily written) semi-naif stumbling onto conspiracies and dueling with Lang’s wife and mistress (Olivia Williams and Kim Cattrall, both sharp). What actually happens is less important than the barest glimmers of that old Polanski magic: ambient paranoia (aided by the Cul-de-Sac–y land’s-end setting) and uneven power struggles (one involving a very crafty Tom Wilkinson as an old Lang associate). The wrap-up is one strange, ah-fuggit mess, on top of Google-powered plot moves, but Polanski’s work therapy could have been a lot worse.