The book begins with the Paris World's Fair of 1878, which Declan Broekhart and his wife are attending. They are there mainly to see a new hot air balloon, which they are to take a ride in. While they are in the air, along with Victor Vigny, the balloon is shot at by men from the ground. During the forced landing, Conor Broekhart is born, flying over Paris.
In the 1890s Conor and his family live on the sovereign Saltee Islands, off the Irish coast, which are ruled by King Nicholas Trudeau. Nicholas is a progressive leader who is helping the islands adapt to the industrialized world.
In a fire accident, when they are trapped on tone of the tower's roofs, Conor saves Isabella (Nicholas' daughter) and himself, by making a makeshift glider and gliding down to sea from the royal palace. After they are rescued from the ocean, King Nicholas knights Conor for his bravery.
Conor spends his days with his tutor, Victor Vigny, studying, and learning how to fly, and exploring the castle with the king's daughter, Princess Isabella. Conor and Victor are obsessed with building a "flying machine". In secret, the head of the island's guards, Marshall Bonvilain, conspires to overthrow Nicholas and seize control of the Saltees. His goal is to turn the islands into a market for the diamonds mined by inmates on the prison island, Little Saltee. Despite Conor's attempt to intervene, Nicholas and Victor are killed by Marshall Bonvilain. Marshall Bonvilain takes control of the islands and tells his subjects that Victor conspired to kill King Nicholas and Conor was his "accomplice". Conor is then framed as the "accomplice" and is thrown into jail on Little Saltee, under the alias Conor Finn.
Conor's cellmate, Linus Wynter, an American musician and spy, helps Conor adapt to prison life. Before long, Conor begins to make deals with a guard named Arthur Billtoe and gangsters called the Battering Rams. Linus soon mysteriously disappears, and Conor believes him dead, as he was told Linus was released, and "release" on Little Saltee usually comes only with death.
Two years pass and Conor is 16. He has persuaded Billtoe to plant gardens for inmates to work in. Conor and Otto Malarkey, a Battering Ram, smuggle seven bags of diamonds and hide them in the gardens. Meanwhile, Conor tricks Billtoe in to getting him bed sheets and other materials using an invention and a few good ideas. Unbeknownst to Billtoe, Conor is using these materials to plan a grand escape during the coronation of Isabella, now old enough to become Queen of the Saltees. The coronation and the arrival of Queen Victoria are approaching and Conor constructs a parachute. He persuades Billtoe to suggest that during the coronation, several hot air balloons are filled with fireworks and released from Little Saltee so that the famous Saltee sharpshooters can shoot the balloons for a grand fireworks display. During the coronation ceremony, Conor escapes from his cell and plants himself in one of the fireworks balloons. When his balloon is shot down, Conor activates his parachute and crash-lands in Queen Victoria's yacht.
Conor, a stowaway on Queen Victoria's private yacht, makes it safely to Kilmore, the Irish village overlooking the Saltees. There he encounters Linus Wynter. The pair finds a tower full of aviation equipment that was once Victor Vigny's laboratory. Conor constructs a hang-glider and adopts the persona of the Airman, a flying French swordsman. By night, Conor makes flights to Little Saltee, terrorizing the guards and digging up his bags of smuggled diamonds. Conor's goal is to use the diamonds to start a new life in the United States of America. Conor's plans are interrupted when Bonvilain finds Conor's tower and harms Linus. Linus learns that Bonvilain intends to overthrow the monarchy again by poisoning Isabella and Conor's mother and father. Conor hears this information and decided he must save his father, mother, baby brother, and Queen...