The Baron and Baroness de Watrin, wealthy philanthropists, have decided to donate a famous diamond from their collection, the Pink Diamond, to the City of Brussels. Professor Einstein is to attend this highly scrutinized ceremony. When the day arrives, Arsène Lupin, who intends to steal the jewel, disguises himself as Einstein. Meanwhile, his faithful Grognard “occupies” the real Einstein by walking around the city with him. But then a group of extremists kidnap Lupin, mistaking him for the famous scientist.