Under Fire takes a look at World War I in a thematic way. Initially, it brings out a picture of elderly men observing the war beginning from on high, without a true care in the world as they are tended to in a sanatorium. They discuss whether it will be the last war and who will win while observing the battles going on below.

Down on the front, men are on watch and guarding their trenches. They eat putrid meals and drink terrible coffee while covered in dirt and grime. They discuss the scale of the war and the evil of their enemies while waiting for more battles to occur. Civilians bring mail and ask them about their situation.

The men get their mail and gossip about what the future holds for them, whether they will be sent to another front soon, perhaps in Egypt, or if they will be sent to the French Riviera for a rest.

Voltpatte, one of the soldiers, is severely injured during a battle, nearly losing his ears. He is sent to the rear to receive medical care and is pleased to get a chance at rest for a while. When he returns, however, he expresses outrage at the laziness of the men who prefer to stay in the hospital and shirk their responsibility, but he is told to allow them their rest.

Another man, Eudore, gets a fortnight of leave to spend with his family, and his wife applies for a permit to go to his village and meet him there, but she is denied, and Eudore spends half his time waiting for her to arrive. After traveling to meet her and coming across other soldiers along the way, he ends up with only one night at home, during which they shelter the other soldiers. Ultimately, he has to leave without truly spending time with his family.

A third soldier Poterloo, breaks rank and surreptitiously spends time with German soldiers in German towns. However, he comes across a house where his wife is spending time with German soldiers as well, and he returns to the front distraught and burdened.

Four of a group of brothers are killed during the war, and the others are unfazed by it. They decide that they must kill the spirit of war, but it is too tough a task as everyone is only focused on killing the enemy. The book ends philosophically, debating whether war is something that can ever truly end.