An omniscient (though possibly unreliable)^[5] narrator relates that in 1750s Ireland, the father of Redmond Barry is killed in a duel over a sale of some horses. The widow, disdaining offers of marriage, devotes herself to her only son.

Barry becomes infatuated with his older cousin, Nora Brady. Though she charms him during a card game, she later shows interest in a well-off British Army captain, John Quin, much to Barry's dismay. Nora and her family plan to leverage their finances through marriage, while Barry holds Quin in contempt and escalates the situation to a duel, when Barry shoots Quin. In the aftermath, he flees from the police towards Dublin, and is robbed by Captain Feeney, a highwayman.

Dejected, Barry joins the British Army. Some time after, he encounters Captain Grogan, a family friend. Grogan informs him that Barry did not kill Quin, and that his dueling pistol had only been loaded with tow. The duel was staged by Nora's family to get rid of Barry so that their finances would be secured through a lucrative marriage.

Barry's regiment is sent to Germany to fight in the Seven Years' War, where Grogan is fatally wounded in a skirmish preliminary to the Battle of Minden. Fed up with the war, Barry deserts the army, stealing an officer courier's uniform, horse, and identification papers after discovering the officer is homosexual. En route to neutral Holland, Barry encounters Frau Lieschen, a young German woman whose lover left for the war and never returned. Lieschen feeds and houses Barry. The two briefly become lovers. After leaving, Barry encounters the Prussian Captain Potzdorf, who, seeing through his disguise, offers him the choice of being turned back over to the British, where he will be shot as a deserter, or enlisting in the Prussian Army. Barry enlists in his second army and later receives a special commendation from Frederick the Great for saving Potzdorf's life in a battle.

Two years later, after the war ends in 1763, Barry is employed by Captain Potzdorf's uncle in the Prussian Ministry of Police to become a servant of the Chevalier de Balibari, an itinerant professional gambler. The Prussians suspect that the Chevalier is an Irish spy in the service of the Austrians, and send Barry as an undercover agent to verify this. Barry is overcome with emotion upon meeting a fellow Irishman, and reveals himself to the Chevalier immediately. They become confederates at the card table, where Barry and his fine eyesight relay information to the Chevalier. After he and the Chevalier cheat the Prince of T�bingen at the card table, the Prince accuses the Chevalier of cheating (without proof) and refuses to pay his debt and demands satisfaction. When Barry relays this to his Prussian handlers, they (still suspecting that the Chevalier is a spy) are wary of allowing another meeting between the Chevalier and the Prince. So the Prussians arrange for the Chevalier to be expelled from the country. Barry conveys this plan to the Chevalier, who flees in the night. The next morning, Barry, under disguise as the Chevalier, is escorted from Prussian territory by Potzdorf and other Prussian army officers.