A respectable professor inherits a house and money in Montevideo from his wild sister, on the condition that he himself behaves in a disreputable way.

Summer 1870. Following the French defeat at Sedan. Léon, a soldier in a detachment isolated in the Ardennes forest, is sent in search of water. When he discovers the most peaceful of rivers, he decides to undress and bathe in it. At bend of the river he catches sight of another naked swimmer. It's a Prussian! Both men start bickering a bit: aren't they supposed to be arch enemies? But they soon fraternize. Unfortunately the patrol has not vanished in the haze and they hear it coming. Each man gets hold of his uniform and runs away in two opposite directions. The only trouble is that Fritz the Prussian has donned the French uniform and Leon the Prussian one!