Narrated from the first-person perspective of Franz, a schoolboy living in the Alsace-Lorraine region of France during the Franco-Prussian War of 1870, "The Last Lesson" begins with Franz running late to school. He dreads the upcoming grammar lesson on participles, which he knows nothing about. He considers running away to spend the day outdoors, but resists the temptation and hurries to school.
On his way to school, Franz passes the town hall, where a crowd gathers in front of the bulletin board. For two years this is where the town has received bad news, and Franz wonders vaguely what has happened now. He reaches the school garden out of breath. Through the window he can see his classmates sitting quietly at their desks and his teacher, Monsieur Hamel, walking past with an iron ruler under his arm. Franz had hoped to sneak to his seat unnoticed, but today there isn't the usual clamor of students loudly settling into their desks.
Franz blushes in fright as he enters the classroom. He is scared that Hamel will admonish him, but Hamel speaks kindly as he asks Franz to take his seat. Once he does, Franz notices Hamel is dressed in his best formal clothes, which are normally reserved for special occasions at school. Franz realizes the entire school seems strange and solemn. The back benches of the classroom are full of men from the village, and all of them look sad.
Hamel announces that this is the last lesson he will teach; an order has come from Berlin that only German will be taught in the schools of the Alsace-Lorraine region. Their new teacher will come the next day, so Hamel wants them to be attentive to their last French lesson.
Franz is shocked by Hamel's words. He realizes why everyone had gathered at the bulletin outside the town hall. He considers how he hardly knows how to write, and his knowledge of French will stop where it is. He regrets not putting effort into learning; too often he skipped school. His books, once so heavy, now seem like old friends. He also feels sympathy for Hamel, who has put on his finest clothes in honor of his forty years of faithful service as a teacher and out of respect for the country that is no longer theirs.
Hamel calls on Franz to recite the definition of a participle. He wishes he could recite clearly, but he gets mixed up on the first word. His heart beats and he doesn't dare to look up at Hamel. However, Hamel says he will not scold Franz, who must feel bad enough. Hamel says it isn't his fault: the problem is that everyone in Alsace-Lorraine has the attitude of putting things off until later, but now it is too late. Now the Prussians will mock them for claiming to be Frenchmen while they can't speak or write their own language. Hamel blames not Franz but his parents, who prioritized having him work in the fields to earn more money, and also himself, who sent Franz to water flowers during class, and who gave the class a holiday when he wanted to go fishing.
Hamel says French is the most beautiful language in the world, and they must guard it, because when people are enslaved, if they have their language they hold the key to their prison. Hamel begins the lesson, reading from a grammar book. Franz is amazed at how he understands and takes in everything Hamel says. This continues as the students and Hamel bring focus to every subject. Hamel has the students copy out "Alsace-Lorraine" during the lesson in cursive. Franz considers how courageous Hamel is to continue with his lessons even when he knows he is about to lose everything he knows and be forced to leave for the country the next day.
The church clock strikes twelve. The trumpets of Prussian soldiers sound outside the windows. Hamel stands up, looking very pale and very tall. He begins to address the crowd but gets choked up. He turns to the chalkboard and writes as large as he can: "Vive La France!" He leans his head against the wall and, without speaking, motions to the class that they can go away because the lesson is over.