Rosamond Lehmann is all but forgotten when people talk about the greats of mid-20th-century British literature. Her stories were less complicated but lots soapier than anything by Elizabeth Bowen or Graham Greene. Her tales of girls becoming women of the world are so elemental it's a wonder the English hadn't gotten around to adapting her simple, juicy books sooner than now.

But here we are with "The Heart of Me," a long, runny version of Lehmann's 1954 novel "The Echoing Grove." It is almost a delightfully trashy piece of "Masterpiece Theatre" but insists on going on at an interminable length. By the time it's over, the two sisters at the movie's center are spent. And you'll want to talk some sense into them long after the closing credits have rolled.

Olivia Williams plays Madeleine, the practical sister, who agrees after their father's 1934 funeral to let Dinah (Helena Bonham Carter) stay in her large suburban London manse for a spell. It's during Dinah's stay that Madeleine decides she's going to marry off her untameable sister to a wet noodle called Charles. The engagement lasts all of a scene, but that's due in large part to Madeleine's jealous young husband, Ricky (Paul Bettany), who pops in on Dinah one night and demands she disengage. She does. And you can pretty much guess the rest.

Madeleine's a different story. She's oblivious to it all: Ricky subsidizes Dinah's new flat, he comes home inexplicably late one night after the next, and he's distant yet coolly accommodating.

Honestly, all she need do is have a gander at Bonham Carter's vampiric eyes and sinfully agape mouth. They alone are enough to make you ask: "Has this woman slept with your husband?"

Bonham Carter, of course, was born to play the selfish sexual conniver, inasmuch as she's been playing it her whole career, both on and — if we're to believe gossip — off the set. She gives to Dinah the flippancy a movie this earnestly heartsick demands.

The first half of "The Heart of Me" is sort of a hoot. You know where it's all headed, and you can't wait for it to get there, as the cheap, cruel ironies pile up almost farcically. Then the film abruptly flashes forward 10 years, then lurches back, then forward again, offering scene after dreary scene of what drove Ricky, his wife, and her sister apart. It's a futile pseudo-literary move, like watching someone try to turn skim milk into heavy cream.

"The Heart of Me" is rated R for simulated sex and some innuendo. Running time: 96 minutes.