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To judge from its trailer, the BBC-co-produced I Capture the Castle threatens to be a conflation of two flagging screen genres: the sensitive chick flick and the lushly upholstered Merchant-Ivory costume picture. But fans of the 1948 novel on which it’s based—whose legion includes writers as disparate as J.K. Rowling and Erica Jong—will be happy to learn that playwright Heidi Thomas and television and stage director Tim Fywell, both making their feature-film debuts, have dramatized Dodie Smith’s engaging comedy-drama without coarsening its wit or overstressing its pathos.

The major problem in adapting I Capture the Castle is finding a way to approximate the novel’s captivating tone. All of its action filters through the shrewd, articulate, often ironic sensibility of Smith’s precocious narrator, 17-year-old Cassandra Mortmain, a budding writer. In three journals, Cassandra chronicles the lives of her eccentric, impoverished family members, who, in the mid-’30s, rent a dilapidated English castle. James, her father, languishes in the throes of a 12-year writer’s block, unable to produce a follow-up to his acclaimed first novel. Topaz, Cassandra’s stepmother, is a frustrated aesthete, a former painter’s model devoted to her cranky, fallow mate. Cassandra’s beautiful older sister, Rose, desperately seeks a means of escape from the family’s oppressive indigence, while her brainy younger brother, Thomas, casts a cool eye on the Mortmains’ foibles. Stephen Colley, a courtly retainer devoted to the family and infatuated with Cassandra, selflessly struggles to keep the castle from collapsing, in return for what passes for room and board.

The castle’s owner dies, and the Mortmains, who are two years behind on the rent, fear the worst when they learn that their home has been inherited by Simon Cotton, an American who arrives, with his brother Neil and their outspoken mother, to take charge of his holdings, which include a lavish neighboring estate. The Cottons, particularly Simon, who has written a dissertation on James’ novel, quickly become involved with the peculiar, intriguing Mortmains. Rose sets her sights on Simon, regarding him as salvation from her hardscrabble existence. Cassandra charts the ups and downs of this relationship, the frustrating plights of her father and Topaz, and, at various points, her own entanglements with Simon, Neil, and Stephen.

Even if there were no other reason to see I Capture the Castle, Romola Garai’s Cassandra would make it essential viewing. Although Thomas’ screenplay transcribes excerpts of Cassandra’s journals as voice-overs, it’s Garai’s subtle, resourceful performance that brings the character to life. Plainly dressed and uncosmeticized, Garai holds the movie together. In a role that frequently restricts her to observing and reacting, she uses her enchanting gray-green eyes and lilting voice to imply, rather than manifest, Cassandra’s intelligence, decency, and humor. And Garai is more than capable of handling the intensely dramatic confrontations with Rose and James that arrive in the film’s second hour.

If Rose were as multifaceted a character as Cassandra, no doubt Rose Byrne’s performance would be as impressive as Garai’s. Instead, it’s merely wonderful. A Titian-tressed beauty, Byrne artfully balances Rose’s steely materialism (“I’d marry a chimpanzee if he had money”) with her hunger for romance. Tara FitzGerald is delightful as the generous, free-spirited Topaz, given to pagan naturism and spontaneous expression. And Sinead Cusack makes the most of her few scenes as the supportive but blunt Mrs. Cotton, whose prodding begins to counteract James’ creative inertia.

Overall, the male performers are less impressive. As evidenced in the recent Lawless Heart, few actors embody middle-aged stagnation as convincingly and effortlessly as Bill Nighy. As James, he’s very well-cast. So is Henry Cavill, a strikingly handsome newcomer physically reminiscent of the young Tyrone Power, whose looks flesh out the underwritten role of Stephen. But E.T.’s Henry Thomas, who plays Simon, hasn’t matured into a particularly compelling actor, and potato-faced Marc Blucas is opaquely bland as Neil.

I Capture the Castle is so consistently charming and satisfying, though, that its flaws are easy to disregard. In some sequences, notably the scene in which the Mortmain sisters visit an upscale London department store, the production designs and costumes are unnecessarily and distractingly ostentatious for a character-driven piece. Similarly, the camerawork of Shakespeare in Love cinematographer Richard Greatrex is, at times, too glossily pictorial, and the score might have better served the film if it had been written for a chamber ensemble rather than Dario Marianelli’s full-bodied orchestra. In an effort to satisfy the 340-page-plus novel’s devotees, Thomas attempts to replicate nearly all of its scenes, so that episodes in the book that could have been strikingly cinematic—such as Cassandra and Neil’s nighttime swim in the castle’s icy, swan-inhabited moat—are too briskly staged to realize their visual and dramatic potential. The rather plot-heavy presentation almost makes one wish that I Capture the Castle had been produced as an up-market television miniseries rather than as a two-hour theatrical feature.

Still, having acknowledged these blemishes, one shudders to imagine the damage that a Hollywood production would have inflicted on the property. Surely obtaining funding for such a commercially questionable project would have necessitated appending one of those inane uplifting resolutions that mar most contemporary American movies. Midway through Smith’s book, Cassandra expresses her annoyance at “finishing a novel with a brick-wall happy ending—I mean the kind of ending when you never think any more about the characters.” In this spirit, Fywell and Thomas conclude their story with an ellipsis rather than a period, sending us out of the theater certain to remember the Mortmains, and wishing them well. CP