It is the first in the young adult fiction trilogy Leviathan, set in an alternate history World War I. The Clankers are characterized by their use of mechanized war machines, while the Darwinists are characterized by their use of living creatures evolved specifically for war. The main characters are the teenage son of the Archduke Franz Ferdinand, Prince Aleksandar, and his future love interest, a cross-dressing Scottish girl, Deryn Sharp.
The novel begins at Konopiště castle, the home of Archduke Franz Ferdinand and his family. Fifteen-year-old Alek is in his room late at night, staging a battle with toy soldiers, Clanker Austria-Hungary against Darwinist Britain and France. He is interrupted by two of his tutors, Count Volger and Otto Klopp, who tell him that it is time for him to practice piloting walkers, namely the Cyklop Stormwalker waiting outside. Excited, Alek does not question the unusual timing of the lesson until his tutors' insistance on speed and secrecy becomes too suspicious. After Klopp and Volger explain that Alek's father has ordered them to take him away from home, Alek shouts that they are lying and tries to pull the Stormwalker's distress whistle cord. Alek is knocked out with a chemical by Volger and hears Volger tell him that his parents have been murdered just before he blacks out.The story of the other main character, Deryn Sharp, begins in London. Deryn has fallen asleep studying her aeronautics manual. She and her brother, Jaspert, discuss how she will disguise herself as a boy in order to enter the Air Service as a midshipman. Deryn is not worried about her flying skills, because her father taught her them years ago. Deryn and Jaspert take a bus to the Air Proving Ground, where Deryn and the other candidates are tested for fear of fabrications with two tigeresques.They are then given the opportunity to fly attached to a Huxley, and Deryn is the first to do so. When a storm strikes during her flight, Deryn untethers the Huxley and is left to float freely. She is eventually picked up by the Leviathan.
Alek wakes up and meets Corporal Bauer. Volger explains how Franz Ferdinand and Princess Sophie were poisoned by the Germans, that the murders were blamed on Serbia in order to start a war, and that the Germans plan to kill Alek as well. He tells Alek that they must flee to Switzerland. Alek does not believe him until a German land dreadnaught begins shooting at them. Alek aids the men to escape from the dreadnaught by loading a Spandau machine gun.
Over the next two weeks, Alek learns how to do lots of things he has never done before, like washing clothes, cooking, and cleaning the machine guns. He also practices fencing and walker piloting. During a piloting lesson, Alek learns that Volger has hidden twelve gold bars on the Stormwalker that are part of Alek's inheritance from his father.
Deryn has also learned a lot about the Leviathan in the last two weeks, which she is now serving on as a midshipman (or middy). She has made friends with Newkirk and enemies with Fitzroy, both fellow middies. When the Leviathan is going to land in London to collect Dr. Barlow, her mysterious cargo, Tazza, and a large trunk, Mr. Rigby tells the middies that some of them will have to leave the ship to make room for the weight of these items. Fitzroy makes the mistake of using bystanders to hold the airship ropes, and is one of the middies who must leave; Deryn and Newkirk are the only two middies on the Leviathan.
Alek, Volger,and Klopp go into Lienz, an Austrian town, to buy supplies for the Stormwalker. Alek gives himself away by correcting the mechaniks shopkeeper when the man tries to sell him an inaccurate model of a Mephisto walker. They must flee without getting the parts that they need, and are pursued by local soldiers. Volger sends Alek ahead to get the Stormwalker, but he is shot at by one of the soldiers. Alek manages to unhorse the soldier, but accidentally kills him. With the help of the Stormwalker, Alek and his men are able to defeat the soldiers. However, now that their general location is known, a land frigate chases them as they travel to Switzerland. The frigate has a secret weapon which it uses against the Stormwalker: phosphorous flares which will stick to a machine and give away its location so that it can be fired at in the dark. Alek cuts off the flare with an heirloom sword and throws the sword away from the Stormwalker, which causes the frigate to lose the Stormwalker in the dark.
Back on the Leviathan, Dr. Barlow almost figures out Deryn's secret, but instead guesses that Deryn is younger than sixteen (the age required to be a middy). Deryn confesses that she is fifteen, and Dr. Barlow believes that this is Deryn's only secret. Soon afterward, German aeroplanes attack the Leviathan, and Deryn is called to feed the fléchette bats. Though they are able to fight off the aeroplanes, the Leviathan still crashes in the Swiss Alps. Deryn is tied to Newkirk and Mr. Rigby, and when Mr. Rigby is shot and falls off one side of the airship, she jumps off the other side to prevent Newkirk from having to cut Mr. Rigby loose.
Alek and his men reach Switzerland, and plan to spend the war in the castle that Franz Ferdinand bought for Alek to hide in. Volger tells Alek about a papal scroll that Franz Ferdinand obtained which gives Alek a claim to the throne of Austria-Hungary. They then hear the Leviathan crashing. Alek goes to help the crew of the crashed airship, against Volger's wishes. He finds Deryn (who he believes to be a boy), and revives her with smelling-salts from his first aid kits. Deryn becomes suspicious of Alek when his story of being a Swiss villager is contradictory, and he pulls a pistol on her when she asks him to come talk to her superiors. Because he is afraid of the hydrogen sniffers, Deryn is able to capture Alek and hand him over to Mr. Roland. She goes back to Dr. Barlow, who is upset because her secret cargo of eggs has been partially destroyed by the crash. Only three of the eggs have survived. Deryn tells Dr. Barlow about Alek, and Dr. Barlow surmises that Alek is Austrian because of the symbols on the first aid kits.
Deryn takes Dr. Barlow to meet Alek, who tells him that she knows how to find his "family." When Dr. Barlow goes to talk to the captain, Alek goes with Deryn to the machine room, where the eggs are being kept. Deryn finds out that Alek is an orphan. When Deryn is about to go up in a Huxley to find Alek's people, Captain Hobbes makes an announcement that there is not enough hydrogen or food for the Leviathan to reinflate. After she is in the air, Deryn notices the Stormwalker heading towards them, and, after sending a message to the crew with semaphore flags, goes back to get Alek to stop the Stormwalker. She pretends to hold a knife to his throat to get their attention, and Alek tells her to ask for Volger.
Alek and Volger meet with the captain and other officers, and Dr. Barlow. Dr. Busk explains how the Leviathan needs food in order to leave the Alps, and Alek agrees to provide some from the castle's storerooms in return for his freedom. The Leviathan's crew keeps Volger as insurance, and they trade Dr. Barlow to Alek in exchange. She brings Deryn and Tazza along as well. They stay in the castle overnight as Alek, Klopp, Hoffman, and Bauer load food on the Stormwalker as Dr. Barlow tells them which foods to bring. As they are traveling back to the Leviathan, zeppelins attack it, and Alek decides to fight the zeppelins and defend the airship. During the attack, the Stormwalker is unable to stand upright, and the engines of the Leviathan are destroyed. Alek offers the engines from the Stormwalker in exchange for passage on the Leviathan, because he and his men cannot stay in the castle now that the Germans have found them. Alek confesses to Deryn who his father is, but does not tell her that he is the heir to the throne. After telling her about his parents deaths, he hugs her, and she starts to have a crush on him.
After Alek and his men have helped the Leviathan's crew install the engines on the airship, the eight-legged walker Herkules heads toward the Leviathan, and the ship must leave. When the ship is far heavier than expected, Hoffman tells Alek about the gold that Volger smuggled aboard. Alek goes to find the gold in Volger's cabin, and, with Volger's help, throws out all but one of the bars. The Leviathan is then able to escape and head for the Ottoman Empire.
- Characters, creatures and objects which appear in Leviathan on Leviathan Wiki