Melanie is a sad lady. She's not particularly attractive and she's a pushover with everyone she knows. They know she's 'nice' and everyone takes advantage of her--her neighbor, her boss...even her own family. It's been this way her whole life. However, when her horrible cousin and her no-good friends decide to play a horrible 'joke' on her, Melanie snaps. No more Miss Nice...now she's out for herself and to give everyone a bit of payback.
I generally liked this film, but must say that it was a bit uneven. Some of Melanie's actions are funny and easy to justify, though her cruelty towards animals is odd and disturbing. Being cruel towards the jerks around her was easy to enjoy...seeing her keep a cat in a covered trashcan for a couple days and tie a dog's leash to a bus' bumper seemed over the top and didn't fit with the character. Still, on balance it was quite good.
By the way, another French film that has some similarities but is far superior is "Tatie Danielle". This wicked old lady is MUCH worse than Melanie but NEVER was nice. And, she's SO over the top mean that the overall film is hilarious and quite entertaining.