After the death of his sister, Canadian teenager Robert Ross signs up for the army to fight in the World War I. Although World War I is a primary focus of the book, The Wars also focuses on more personal wars, such as the battle Robert is fighting within himself. It explores the loss of innocence on several levels, both before Robert goes off to war and during the fighting itself. The structure of the novel is unique and alternates between first, second and third person points of view.
A young man named Robert Ross is introduced as squatting in a tattered Canadian military uniform, with a pistol in hand. A nearby building is on fire, and a train is stopped. There is evidence of war, and Ross is shown to be in the company of a black horse and a dog. Robert, the horse, and the dog seem to have been together for a while, as they understand each other. He decides to free a herd of horses from the train, and the prologue ends with the horses, rider, and dog all running as a herd.
Robert Ross enlists in the army to escape the guilt he feels after the recent death of his sister, Rowena, who died from falling out of her wheelchair while playing with her beloved rabbits in the barn. Robert feels guilty because he was unable to save her since he was making love to his pillows in his bedroom when he should have been watching her. His mother orders Robert to kill the rabbits after Rowena's death; when he refuses, his father hires Teddy Budge to kill the rabbits. In an attempt to stop Teddy, Robert is beaten up. Later, while he soaks the resulting bruises in the bathtub, Robert's mother comes in to talk to him. Drunk and visibly upset, she says that she knows Robert wants to go to war, and she accepts that she cannot stop him.
Robert goes to army training. There, he meets Eugene Taffler, a lauded war hero. Taffler shows him how to break bottles with stones, prompting Robert to think of him as David throwing stones at Goliath. Robert then goes with his soldiers-in-training to a brothel named Wet Goods . He goes into a room with the prostitute Ella; when she realizes that he has accidentally ejaculated in his pants and therefore will not be having sex, she decides to pass the time by showing him a peephole into the next room. Here Robert sees Taffler having sadomasochistic sex with another man. Out of anger, Robert throws his boots at a mirror and a water jug, scaring Ella.
While sailing to England on the S.S. Massanabie , Robert must kill a horse that broke its leg. While struggling to kill the horse, he fires and misses many times before landing his shots.
Robert is now in France and in charge of a convoy. While scouting ahead in the fog, he falls into a muddy sinkhole and nearly drowns. After saving himself, he is met by Poole and Levitt, two of his men. They eventually reach the dugout that will be their temporary home. There, they meet the three other men that live in the dugout: Devlin, Bonnycastle, and Rodwell.
Rodwell cares for injured animals he finds: birds, rabbits, toads, and hedgehogs. The rabbits painfully remind Robert of Rowena. However, Robert manages to build a bond with Rodwell, the only other civil soldier who cares and respects animals.
Robert remembers Harris, another soldier whom he had befriended on the ship when both fell ill. On land, Harris died two days before Robert was scheduled to leave for France. It is during their time in the infirmary together that Robert and Harris meet Taffler again. After Harris dies, Robert asks Taffler to help him in burying his friend. However, Robert learns that Harris has already been cremated. Disappointed by this turn of events, Robert and Taffler dump the ashes into the sea. Robert says, "This is not a military funeral. This is just a burial at sea. May we take off our caps?"
On February 28, 1916, the Germans set of a string of land mines, strategically placed along the St. Eloi Salient. The whole countryside goes up in flames. This is the second half of a battle that the Canadians thought was already over. "30,000 men would die and not an inch of land would be won."
Robert is now experiencing trench warfare at its worst. Following a shelling of the dugout, his fellow soldier Levitt loses his mind, and Robert finds himself close to the brink.
Ordered to place guns in a location sure to be a deathtrap, Robert and his men find themselves on the wrong end of a gas attack in the middle of a freezing cold winter. Robert is instructed to place the guns in a crater that is formed by the shelling attacks. As he descends the side of crater, Robert slips and smashes his knees on a discarded machine gun, creatin the men start descending the side of crater, there is a sudden gas attack. The bottom of the crater is full of freezing water, and many begin jumping into it. Robert takes control with his pistol and instructs the men to urinate on strips of clothing and hold them over their faces. One man is too scared to urinate, and Robert must do it for him. The men then lie down, feigning death. Hours later, Robert finally sits up and surveys their surroundings. He immediately realizes that they are being watched by an enemy German soldier sitting at the lip of the crater. Rather than shooting the soldiers, the German allows all of Robert's men to escape the crater. Just as Robert is leaving, however, the German makes a quick motion, and Robert shoots the German. Thinking that the German had been reaching for his rifle, Robert is shocked when he realizes that the German was only reaching for a pair of binoculars to look at the bird flying overhead; Robert is even more horrified to see that the German had had a sniper rifle, meaning that he could have killed Robert and his men if he had wanted to. Robert hears a bird chirping above him and, from then on, is haunted by the sound.
In an interview, Juliet d'Orsey explains that the d'Orsey home was converted into a hospital for soldiers during the war. It is here that Robert recovers from his own injuries sustained in Part Three. Once again he meets Taffler, another patient at the d'Orsey home; Robert is devastated to find that Taffler has lost both of his arms in the war. While Taffler seems to be in good spirits, he ends up attempting suicide; one day, as Juliet comes into Taffler's room to give him some flowers, she finds him trying to rub his raw arm stumps against the walls and subsequently bleed to death. Various people, including Robert, respond to Juliet's screams, and Taffler is saved.
Juliet tells Robert that his assigned room is haunted by the ghost Lady Sorrel, who comes into the room every night and lights the candles. One night, Juliet thinks it would be a neat prank to dress up as Lady Sorrel, walk into Robert's room, and light the candles. When Juliet opens the door, she sees Barbara and Robert Ross having sex, so violently that Juliet at first thinks that Robert is hurting Barbara. Although they do not see who opens the door, Juliet begins to feel guilty. When Robert eventually leaves the home, Juliet slips him a candle and a box of matches in an effort to explain herself and apologize.
Robert heads back to battle on a small train. He gets hopelessly lost on the way and loses his pack. After many weeks of travelling in circles, he arrives at Désolé, a mental institution. In the bath house, he is brutally raped by four men. Although he assumes that they are patients of the institution, he is horrified to learn that they are fellow soldiers. When he returns to his room, he finally receives his lost pack. He burns his only picture of Rowena, as an act of charity, reasoning that it would be horrible for something so innocent to exist in such a perverse world.
Robert then returns to the front. The Germans begin firing shells. Robert asks Captain Leather to let the horses out of the barn, as they will die if the barn is hit, but Captain Leather refuses. Robert returns to the barn and asks his friend Devlin to help him release the horses. As Devlin runs out to open the gate, Captain Leather comes out of hiding and shoots Devlin dead. He then fires at Robert but misses. At that moment, three shells land. Soon everything is burning around Robert; even the horses are slowly burning alive. Robert finds Captain Leather and shoots him dead.
Robert runs away, as he knows he will be court-martialed for disobeying orders. He finds a black horse with a black dog beside an abandoned train. Before riding the horse down the track, he realizes that there are horses in the train. He frees 130 horses from the train and flees the area with them. As Robert is riding with all the horses, a soldier stops him and tries to force him to return the horses; Robert shoots him dead.
Robert is finally caught in a barn with the horses. The soldiers surrounding Robert set the barn on fire in an attempt to smoke him out. However, the doors of the barn are locked. Before Robert can open them, the roof collapses on him and the horses. Robert is saved but badly burned, and all the horses and possibly the dog are killed.
Robert turns down an offer of euthanasia from a nurse from Bois de Madeleine hospital before being sent to England and tried in absentia . Since he could not be kept in prison, he was given leave to stay in St Aubyn's for longterm treatment.
Juliet d'Orsey remains by Robert's side until his death in 1922. Mr. Ross is the only member of his family to come see Robert buried.