François is coming home on his 40th birthday, for which his wife has secretly prepared a surprise party with old friends with whom he used to fence.

He is called back by his colleague Albert for a mission involving planting a microphone in an arms smuggler's car.

After infiltrating her apartment and planting a camera, they discover she is helping a missile expert and the smuggler to meet.

Hélène then meets Simon, who pretends to be a secret agent while, in reality, he is a car dealer living in a caravan.

They thwart the arms dealer's plan of blowing up a football stadium in Paris and all ends well.