During wartime, Isabel Green struggles to manage the family farm with her three rambunctious children while her husband is away fighting. To make matters worse, her demanding and wealthy niece and nephew from the city come to stay with them, adding to the chaos. As the children's behavior spirals out of control, Isabel finds herself overwhelmed by the mounting pressure, and it seems like there's no end in sight to her problems. Just when she thinks she can't take any more, a mysterious and magical figure, Nanny McPhee, appears on her doorstep. Nanny McPhee uses her extraordinary abilities to instill discipline and order among the children. Through a series of enchanting and often humorous lessons, the kids gradually learn important values such as cooperation, kindness, and responsibility.

As the story progresses, Isabel faces additional challenges from her scheming brother-in-law, who is determined to sell the family farm for his own gain. Nanny McPhee's presence not only helps the children but also gives Isabel the confidence to stand up for herself and protect her family's legacy. Through teamwork, perseverance, and a little bit of magic, the children and Isabel find a way to save the farm and restore harmony. Along the way, they discover the importance of family, friendship, and believing in the impossible.