Victor is a screenwriter whose last work about the Marechal Pétain is refused by his producer. To add insult to injury, he tells Victor he is paunchy. The unfortunate man becomes obsessed by his weight. He goes out of his way to lose pounds, abetted by his wife, Corinne, who puts him on a reducing diet. He must also go to a health center where he takes exercise. Nothing really works. And one day, in a brewery, he cannot withstand his diet anymore and he has a gastronomic meal. Desperate, he tries his luck at a weight watchers reunion.

At the end of the Great War, Arsène Lupin resumed his adventurous life. Theft of three paintings committed by an adversary leads him to the treasure of the Golden Fleece.