In “Smut,” a devilishly charming pair of novellas, Alan Bennett introduces us to two fusty middle-aged women and sets them on a course for reinvention. But there’s no mistaking his territory for Oprah’s.

Bennett, the British author and dramatist best known for “The History Boys,” has something far more subversive – even obscene – in store. His matrons are not at all what they seem, and in pushing them toward revelation, he playfully shows us how misleading appearances can be.

The eponymous star of “The Greening of Mrs. Donaldson” is a recently widowed mother who finds herself in need of both money and company. To cope, she rents out a room in her house and gets a job at a hospital demonstrating medical conditions to students. “It’s a way of not being yourself,” she says of her new occupation. Before long, Mrs. Donaldson becomes the hospital’s answer to Meryl Streep, acting out all manner of maladies. At home, her life takes a less dramatic but equally remarkable turn when her lodgers propose an unusual form of rental payment.

It is here that Bennett’s book turns frankly pornographic, as frumpy Mrs. Donaldson transforms into “a leering loose-breasted attendant on the pleasures of others.”

These farcical bedroom scenes might at first seem unlikely, even gratuitous, but the ever-cheeky Bennett is challenging our expectations as much as his characters’. Mrs. Donaldson makes for a silly voyeur, distracted by the dust on the floor, comparing sexual positions to vases she’s seen at the British Museum and at one point aiding “the proceedings” by steadying the headboard in an effort to save a table lamp that was a wedding gift. Her secret life nonetheless emboldens her, “sheathing her against the petty annoyances at the hospital” and “the bullying of her daughter.”

Bennett could have left it there, as a cautionary tale in which “the lady grieves for the sound and sensible woman she had once been and been thought to be,” but he has more sinister plans for Mrs. Donaldson, the details of which can only justly be revealed by reading this arch story.

Mrs. Forbes, the heroine of the book’s second, slimmer novella, “The Shielding of Mrs. Forbes,” appears to be prim and uptight and, though not a widow, is often taken for one: “She had so much the air of a woman who was coping magnificently that a husband still extant took people by surprise,” Bennett writes.

Mrs. Forbes is grieving, but for her son, the handsome Graham, who has “chucked himself away” by marrying a woman she deems too plain for him. A blackmailer, though, is eager to tell her the truth about her son’s duplicitous double life. The vain Graham and his priggish mother seem destined to get their comeuppance, but Mrs. Forbes has secrets of her own that make the title of this delightfully sly comedy especially ironic.