Melissa McCarthy and Richard E. Grant in Can You Ever Forgive Me? (Fox Searchlight)

 That lady on the north side of 40 who you see out of the corner of your eye, trudging along in a New York City winter with that behind-on-the-rent look: Do you ever wonder what her story is? Does her air of worry make you feel guilty?

Lee Israel (Melissa McCarthy) is one of these women. Lee may look like a loser, but she’ll verbally cut a bitch and curse you for your pity, too. She’ll make scary crank calls to your house, steal toilet paper from your party, hoodwink New York’s most snobbish connoisseurs, and have a ball doing it. That’s her story. And that story is Marielle Heller’s Can You Ever Forgive Me? one of the most entertaining movies in a long, long time.

Dark comedy can be tough to pull off. Heller’s first feature after The Diary of a Teenage Girl goes to bleak places with an F-you sense of humor that cuts the harshness. The script is based on the bizarre tale of the actual Lee Israel, a best-selling biographer who switched from subjects like Tallulah Bankhead to Estee Lauder and watched her sales (and pride) collapse in the mid-1980s. Desperate for money, Israel started forging delightfully droll letters from the likes of vintage stars Noel Coward and Dorothy Parker and selling them on the memorabilia market. Her bon mots sparkled too brightly; once they started attracting suspicion, Israel took to stealing the real thing.

As it traces Israel’s life of crime, the film gives comedienne Melissa McCarthy a role that expands her onscreen acting range. For all her outsider toughness, Israel can still be stung by rejection or find herself unnerved by the loneliness that unexpectedly creeps up after years without a date. McCarthy takes Lee through sardonic glee to vulnerability, rage, and grief in a performance that somehow becomes more affecting by her dumpy wig and dowdy wardrobe.

McCarthy’s well matched by Richard E. Grant as accomplice and fellow drunk Jack Hock. One wonders what devilish fun Grant might have had with the arch lines in Rupert Everett’s current The Happy Prince; the actor knows how to deliver Wildean gay wit with a sheen of lowlife sleaze. Lee and Jack meet over drinks way too early in the day at the landmark gay bar Julius and quickly form a wary symbiosis based on loneliness, booze, and an off-kilter sensibility not always appreciated by those around them.

We watch Lee come up with her brilliant forgery idea, then deceive effete dealers and rip off naïve archivists until she gets caught. The film’s plot is suspenseful but slight. That’s fine, because it allows Grant and McCarthy to play off each other and settle into their personas as their characters careen through a dimly recalled New York of dusty bookstores, elegant antiquarian markets, and gloomy tippling holes. Writers Nicole Holofcener (Please Give) and Jeff Whitty, whose fizzy and fun Head over Heels is currently playing on Broadway, nail how lesbians and gay men rag on each other when straight folks are out of earshot. Lee and Jack get gleefully hooked on their scam, so when these two fall out, it’s almost as sad as the death of Lee’s beloved cat. The film has a real heart beneath its defiant repartee and comic take on underdogs in a jam.

A few details: Can You Ever Forgive Me?‘s jazz score feels a little too well-behaved for a film with such antic mood swings, and it comes as a relief to hear an gnarly blast of rock when Lee begins to panic as her scheme unravels. The production team has done an insightful job evoking scabby barrooms and run-down, cat shit–spackled interiors. And finally, Jane Curtin appears in a spot-on cameo as Lee’s exasperated agent, the kind of high-toned New York doyenne who can deliver an acrid put-down in style.