This time ''Nanny McPhee'' arrives to help a young mother played by Maggie Gyllenhaal - while her husband is away.
By Tim Masters
Entertainment correspondent, BBC News
Emma Thompson is no fan of the old showbiz chestnut "never work with children or animals".
"I refute that!" she cries out, during an interview about her new film Nanny McPhee and the Big Bang - which involves a lot of both.
Thompson, who also wrote the screenplay, reprises her role as the snaggle-toothed nanny with magical powers who comes to the aid of a family in need.
Both Maggie Gyllenhaal and Rhys Ifans adopt new accents in their roles
In this sequel to the 2005 film, the action has jumped from the 19th Century to a farmhouse during World War II. The film also stars Rhys Ifans, Maggie Gyllenhaal, Dame Maggie Smith, Bill Bailey and Ralph Fiennes in a cameo role.
"With children you need a lot of energy on the set, because sets can be rather airless and childish spirits can lose energy," says Thompson.
"My job on set is to keep them energised, so it's an exhausting process, but the rewards are indescribable.
"When a child or animal does something utterly spontaneous and you capture that on film you have pure gold."
Thompson admits to a piece of spontaneity herself - involving some synchronised swimming piglets. It came during a scene in which the children are watching the computer-generated farm animals dancing in a lake.
Thompson takes up the story: "They didn't have anything to react to because we hadn't done the CGI, so I said to the first assistant director 'drag me into the water and pretend to drown me'.
"So when you watch the children laughing hysterically it's because I'm being pushed underwater by a member of the crew!"
Maternal wisdom
Spoilt brat Celia (Rosie Taylor-Ritson) provides an extra challenge to Nanny McPhee
Maggie Gyllenhaal plays Mrs Green, the harassed mum who has lost control of her children and two snobby cousins who have arrived from London.
"She's a mother who's totally overwhelmed, who's failing all over the place, she literally has a bird's nest in her hair," says Gyllenhaal, who perfected a British accent for the role.
Gyllenhaal, who became a mother in 2006, adds: "For so long people weren't comfortable with women being both professional people and mothers - so we had to act like we had it all together and we could do everything perfectly.
"Now people are starting to open up a little bit about how hard it is to be a mother, about how every child brings you to your knees."
Did Emma Thompson - herself a mother - impart any wisdom on set?
"I was working on this movie for four months with my daughter - and no nanny," says Gyllenhaal. "Emma said 'you will drop the ball sometimes, that's how it is being a mother and a woman'. That was one of the best pieces of advice she gave me."
Mud bath
Glorious mud: Filming took place during one of the UK's wettest summers
Although the film's farm scenes are bathed in sunshine, the shoot took place during a wet British summer.
The cast recall muck aplenty.
Gyllenhaal laughs: "Once I fell down flat in the mud in a completely pristine white wedding gown - which was not the intention."
Rhys Ifans, who plays villainous Uncle Phil (complete with West Country accent), adds: "It was like Glastonbury!"
The rain was an additional challenge for the film's director, Susanna White, an experienced TV director working on her first full-length movie.
She had first read the Nanny McPhee script while filming Iraq war drama Generation Kill in Namibia.
"It was completely mad for my first feature to take on children, animals, CGI, and the English weather," she laughs.
"Sitting under a dripping tarpaulin, there were times that I wondered what on earth I'd done and I longed to be back in the sun of Africa."
Pig Floyd
Fans of Pink Floyd may be interested to know that White has added a visual nod to the band's Animals album cover during the scenes set in war-time London.
As Nanny McPhee speeds past Battersea Power Station on her motorbike, among the barrage balloons in the sky is one that is distinctly pig-shaped...
"You are right!" says White, when challenged over the in-joke. "I hid various pigs throughout the film. I am a Pink Floyd fan..."
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