A man named Ted Narracott buys a colt for 30 guineas when he was supposed to buy a horse for plough at an auction. Ted's son, Albert, names the horse Joey and grows to love him, protecting him when Ted is drunk. While with the Narracotts, Joey also meets Zoey, a horse who was a source of comfort to him, and whose name partially inspired his.
Ted sells Joey to the army before Albert can stop him. Albert tries to sign up for the army, but he is too young but promises to come back for Joey. Joey is trained for cavalry service by Corporal Perkins, and Captain James Nicholls is his original rider, leading a unit of mounted infantry. Joey soon befriends Topthorn, a horse ridden by Captain Jamie Stewart. During a charge against the Germans, Nicholls is killed. Stewart assigns Trooper Warren, a nervous young man who rides heavier but is kind, to ride Joey.
During another charge, Topthorn and Joey carry Warren and Stewart into the enemy lines, and are the only two of many, but they are captured by the Germans. They use Joey and Topthorn to pull an ambulance cart for the hospital, where the two are famous and respected for saving many lives. The Germans allow Emilie and her grandfather, who live in a farm near the front lines, to care for Joey and Topthorn. Emilie grows to love the horses like Albert loved Joey, caring for their injuries and feeding them every night. Soon, the Germans move their hospital somewhere else, and Emilie and her grandfather are allowed to keep the horses, who they use for their farm. Topthorn was not bred to plow, but learns from Joey, who has experience from the Narracott farm.
Soon, however, Germans pass by their farm, and take away the horses to pull their artillery wagon. The two meet Friedrich, who befriends them and tries to care for them, growing to love Topthorn and telling them that he did not want to be a soldier. Joey and Topthorn are two of the last few survivors of the artillery-pulling team. One day, after drinking water with Joey, Topthorn dies from heart failure. The Allied artillery starts shelling right after the Germans and Friedrich is killed. After seeing an Allied tank for the first time, Joey runs in terror and is wounded by barbed wire before breaking free. Both the Allied and German soldiers see the wounded Joey in no-man's-land, and a British soldier wins possession of him by flipping a coin with a German soldier and winning. However, their few minutes of friendly peace create a bond between the two before they separate.
At the veterinary hospital, Joey happens to be cared for by Albert, who works there and has a friend named David. Albert realizes that Joey is his old horse only after cleaning all the mud off him, and seeing how he responds to his whistle. Albert starts caring for Joey again like he used to. Later, David and two horses from the hospital are killed by a stray shell, putting Albert in a state of depression, as David had cared for him like a brother. At the end of the war, Major Martin announces that they will auction off all the horses, despite the protests of Sergeant Thunder and the rest of the soldiers. During the auction, Sergeant Thunder loses to an old man for Joey. The man is Emilie's grandfather and was looking for Joey. Emilie's grandfather tells Albert about how Joey and Topthorn came to their farm, and that Emilie had lost the will to live after they were taken from her, with Emilie fading away and dying at just 15 years old. Emilie's grandfather sells Joey to Albert for a cheap price, in return for telling people about Emilie, and keeping her memory alive. Albert and Joey return to England, where they live in peace and Joey meets Albert's girlfriend, Maisie, with whom he does not get along very well.
