Just when you thought the whole British nanny thing had crested, that naughty stools and one-hour family makeovers were on their way to the cultural dustbin, in walks a quiet, grim, borderline-grotesque woman in a faded black frock. She is Nanny McPhee. Aware but unenamored of her illustrious Hollywood predecessors, she flies in accompanied not by a talking umbrella but a homely walking stick; she counsels spoonfuls of sludgy brown elixirs instead of sugar; she wouldn't think of breaking into song. She is the anti-nanny, or perhaps the nanny we deserve.

"I think she would not necessarily approve of Mary Poppins," guesses Emma Thompson, who should know, having written "Nanny McPhee" and starred in the title role. "Nanny McPhee would think that getting large lamps out of carpet bags was a little bit show-offy. And helping children to tidy up their rooms by getting things to jump in the drawers would perhaps not be the thing that engendered most a sense of responsibility."

By phone from New York, Thompson says that responsibility, and the idea that children do not misbehave without motive, were central to her conception of the film, which is very loosely based on Christianna Brand's "Nurse Matilda" books from the '60s and '70s. In Thompson's re-imagining, the Brown family — a father (Colin Firth) and his seven children — descends into wholesale chaos after the death of the children's mother. The little tykes have taken to, in contemporary parlance, acting out, which here takes the form of well-worn schemes to scare off potential nannies.

Nanny McPhee is a completely shocking creature whose face is a funhouse-mirror version of Thompson's, a humongous nose orbited by unsightly warts and all manner of gruesomeness. She is not especially cruel, but neither is she especially kind. After all, these are not the sort of kids — unlike their '60s counterparts — who need to learn how to have fun. They are children of our own Edwardian era, Thompson says, "kids who know how to have fun but have lost their father in a different way."

"When they first put the stuff on my face, I thought, 'What the hell am I going to do with this? How am I going to move my face?' " she says. In part by design and in part out of necessity, McPhee became a serene, often silent presence. "It was almost like masque work. ... She creates space and a calm in which people can work things out for themselves. It's the best kind of parenting, actually, because it's consistent and judicious, and it's not unnecessarily kind."

Unnecessary kindness is one of the occupational hazards of being a parent, according to Thompson, and also of being British. "As a British woman, I've been brought up to be kind all the time, that kindness is of paramount importance."