Six soldiers and their lieutenant are on an exploratory mission a few days before the end of a training course. Unfortunately, due to a mistake, the group gets lost and accidentally ends up on French territory. The lieutenant is then faced with a real dilemma: returning to Switzerland via the nearest customs post means accepting the legal consequences—in fact, any Swiss national who crosses the border with an assault rifle is automatically considered a deserter—or returning to the country illegally through the forest, which is foreign territory and sometimes dangerous.

Henry Dunant, son of a Geneva francophone upper class bourgeoisie family, works for a Swiss exploitation company in French Algeria; when the colonists are thirsty, he returns determined to convince the firm and emperor Napoleon III to build a dam for them. After his Uncle, Dr. Hubert Dunant, diagnoses him not with Algerian typhus, just malaria, also his first meeting -dropping drawers in hospital for a shot- with nurse Cécile Thuillier, and meeting his careerist brother Daniel's fiancée, Léonie Bourg-Thibourg, daughter of the firm's boss, the board approves his plan. On his way to the emperor, who didn't even concede to receive him, Henry gets stuck in Castiglione, part of the Austrian province Lombardy which French troops came to 'liberate'; his Geneva friend Dr. Louis Appia saves his life by presenting him to suspicious Austrian troops as his medical assistant...