The seven Brown children – Simon, Tora, Eric, Lily, Christianna, Sebastian and Baby Aggy – may well be the naughtiest children in the history of the world. Their beleaguered father, Mr. Brown, has his hands full taking care of his troublesome offspring by working long days at the local funeral parlour. The children’s mother died only a year ago but Mr Brown’s imperious Aunt Adelaide, who supplements his inadequate wages, has threatened to cut off her allowance to the family unless Mr. Brown remarries within a month. Debtor’s prison awaits him if he doesn’t comply, and the fate of the children would be unimaginable.
Mr Brown decides not to tell the children but they find out and assume he doesn’t care enough about them to tell them they’re about to have a new stepmother. As a result, their behaviour worsens, and their acts of outrageous mischief send yet another nanny screaming from the house. Simon, the oldest boy and the leader of the pack, keeps a chart showing the amount of time it’s taken to drive away each of their 17 nannies along with the act of mischief that did the trick
One night, as the children are wreaking havoc in Mrs. Blatherwick’s definitely-off-limits kitchen, there appears at the front door the legendary Nanny McPhee – a stern and comically ugly little woman whose features include a bulbous nose, a single repulsive eyebrow, a pair of hairy warts, and a particularly unsightly snaggle tooth. Mr. Brown has doubts about this person he supposedly needs, but finds himself unable to give satisfactory answers to her questions about his children. “Do they say ‘please’ and ‘thankyou’?” she asks. “In what context?” is his weak reply. Nanny McPhee makes her assessment – “Your children need me” – and finds her way to the kitchen where she encounters for the first time the dreadful behaviour of the Brown children. The children are somewhat put off by this creature with the alarming appearance, but they pretend not to see or hear her and defiantly resolve “to play in the kitchen all night long.”
But one bang of Nanny McPhee’s magic stick changes everything. Suddenly the children’s antics are speeded up beyond their control and they realize they’ll have to play in the kitchen all night long, whether they want to or not, unless they ask Nanny McPhee to let them stop. A battle of wills takes over between Simon and Nanny McPhee as to whether or not he will say “please,” but when it looks as though he’s on the verge of getting “Cook blown up and Aggy boiled,” Simon relents and says the word he never says. And says it politely.