O'Brien takes the reader through a typical day in the life of a soldier in Vietnam. We are briefly introduced to a small number of fellow 'grunts' and the commanding officer of Alpha Company, the rifle company O'Brien was assigned to, one Captain Johansen. (Names and physical characteristics depicted in the book were changed.)
Rather than proceed chronologically, O'Brien takes the reader back to the beginning of his induction into the US Army. The reader learns about the author's home town, Worthington, Minnesota and to which O'Brien moved when he was 9 years old. We are led through his childhood, playing various army games, and learning about World War II from returned veterans and the Korean War which was taking place at the time.
The story of his tour itself continues to unfold while the reader is simultaneously taken through O'Brien's training at Fort Lewis, Washington, where he acquaints a man of similar situation named Erik. Together, the two decide to engage in a psychological resistance against the government.
After debating over the idea of desertion, O'Brien arrives in Vietnam in 1969 and spends a week at a base in Chu Lai (home to the Americal Division from approximately 1967 until 1971), receiving last-minute training such as mine sweeping and grenade throwing as well as the essential do's and don'ts of jungle warfare, before being sent to Landing Zone Gator in Quang Ngai Province where he is assigned to Alpha company, 5th Battalion of the 46th Infantry, 198th Infantry Brigade.
O'Brien describes his time in Alpha Company and the various events that took place during his time there, as well as some of the people he encountered. Among the scenarios O'Brien describes is one about the various mines that are encountered by the infantrymen, and the indiscriminate way that these devices disfigure and maim both combatants and civilians.
Not long after the accidental shelling of a lagoon village by the A Battery, 1st Battalion, 14th Field Artillery Regiment, that Alpha Company was protecting (near an American firebase), O'Brien is offered a job at the rear and is airlifted away from the fighting, where he encounters a rear echelon officer, Major Callicles (battalion executive officer), who deals with the investigation into the My Lai Massacre committed by the Charlie Company of the same battalion. The memoir ends with O'Brien being flown home.