I Capture the Castle, a dainty coming-of-age story about a naive young woman and her more worldly sister, receives the buttonhooked BBC treatment for reasons that are both mysterious and inevitable. The mystery is, if you want to romanticize 1934 England, with its genteelly brutal class divisions, why turn a 1948 cult novel by Dodie Smith (who went on to write ''101 Dalmatians'') into a film with such staid 1970s public-television production values? The inevitable follows: The sets-and-costumes-loving team that made ''Shakespeare in Love'' couldn't resist the romantic Suffolk setting, castle and all, in which an eccentric, once successful novelist (''Lucky Break'''s Bill Nighy) nurses a massive writer's block, causing his colorful brood to fall into bohemian penury.
''Castle'''s ''I'' is 17-year-old Cassandra (Romola Garai), a budding writer; Cassie adores her more glamorous older sister, Rose (Rose Byrne), a realist hell-bent on escaping her shabby surroundings who seizes on Simon (Henry Thomas), a rich American, as her ticket to a better wardrobe. Garai and Byrne are pleasant, in that clasp-to-the-bosom way we like to think of period-piece siblings, the former's unadorned prettiness contrasting efficiently with the more rococo beauty (and performance) of the latter. Brief scenes with the wonderful BBC drama vet Sinead Cusack as Simon's no-nonsense mother suggest how much more of her gumption the story could use. And Nighy uses his mobile lower lip regularly to convey a selection of discontents.
But the direction by Tim Fywell and script by Heidi Thomas (two Brit-TV pros making their feature film debuts) allow for strangely little cinematic mobility at all. Smith's book is a charmer, but the keys to this ''Castle'' have been misplaced.