The novel takes the form of journal-like anecdotes which the unnamed narrator claims to be writing to record his time in the war. It follows a squad of French volunteer soldiers on the Western front in France after the German invasion. The book opens and ends with broad visions shared by multiple characters but beyond these the action of the novel takes place in occupied France.
The anecdotes are episodic, each with a chapter title. The best-known chapter, "The Fire" (Le feu) shares the French-language title of the book. It describes a trench assault from the Allied (French) trench across No-Man's Land into the German trench.
In contrast to many war novels which came before it, Under Fire describes war in gritty and brutal realism. It is noted for its realistic descriptions of death in war and the squalid trench conditions.