Nanny McPhee slowly teaches the kids a series of rules, and each time they do something nice her appearance changes for the better.
To buy some time, the maid Evangeline is sent away with the Great Aunt in place of one of the kids.
The wedding looks like it'll go ahead, but the kids manage to stop it - and Colin Firth realises that Evangeline is actually hot and great with the kids, so wants to marry her instead.
Nanny McPhee walks her down the aisle, and magically Evangeline looks a million bucks in a white wedding dress, while Nanny McPhee shows her true self, wart-free and all.
They get married, and everyone lives happily-ever-after. Except Nanny McPhee - who skips town to help somebody else, just like the Lilttlest Hobo.