This novel is an alternate historical sci-fi where main characters Alek and Deryn prevent a mad scientist from using his invention to kill innocent people, in a bid to end World War I. Alek is the legitimate heir of the Austro-hungarian throne and he is on the run from the Germans. After his father's assassination in the first book of this series (Leviathan) the Germans try to kill him in order to take full control of the countries under his rule. He and his most loyal men (among them a Count named Volger) successfully manage to elude them. In this world, allegiances are divided roughly into two groups: the Clankers (societies based on machines) and the Darwinists (societies based on genetically modified creatures). Alek is a Clanker but as the story progresses through the first and second books of the series, he becomes more at ease with the Darwinists, who are mostly British. Deryn Sharpe is a young English girl who has disguised herself as a boy in order to become an airshipman like her father. In the first book of the series, she meets Alek when she rescues him and his men and bring them aboard the Leviathan. The Leviathan is an airship that is essentially a giant whale-creature that flies by burning hydrogen produced from a symbiotic relationship with bacteria in its guts. Since meeting each other, Deryn and Alek have become steadfast friends, having survived many adventures together. In exchange that the Leviathan give Alek and his men a lift away from the Germans, Alek helps attach an engine from his broken down Walker (a war machine that moves on two legs) onto the Leviathan, improving the airship's speed and maneuverability. In the second book of the series, Behemoth, Alek tells Deryn that he not just an Austrian noble but the heir of the Austro-Hungarian throne. Deryn still has not told Alek she is actually a girl.
The third and last book of the series begins with Dr. Barlow receiving a message from Czar Nicholas, asking that the Leviathan pick up a package for him. Dr. Barlow is a direct relation of Darwin and because to her extensive knowledge, she is of great importance to both war and diplomatic efforts. During the trip to Siberia, Deryn keeps trying to tell Alek that she's really a girl but keeps "chickening out" at the last minute. Count Volger already knows Deryn's secret and tries to use his knowledge to blackmail her to spy on Dr. Barlow for him.
The Leviathan descends near the Trans-Siberian Trailway in Russia to pick up some top secret cargo for the Czar. The cargo turns out to be machine components. Dr. Barlow asks Alek help her figure out how to assemble the cargo as he has extensive Clanker knowledge. Alek and his men assemble the device and figure out it's a device that detects metal.
As they travel further, they encounter an airship wreckage and a settlement nearby. When they rescue the people in the settlement, they meet Nikola Tesla, a scientist and inventor. Mr. Tesla insists that the Leviathan must take him and all his gear with them but it takes so much time to load everything on that the wild bears catch scent of the Leviathan and try to attack the crew. Mr. Tesla uses his electrical walking stick to fend them off. Later on, they find out that Mr. Tesla had been working on something huge and metal and whatever it is, he's likely already carried it aboard. Mr. Tesla explains that it's a Tesla Canon called the Goliath which is capable of setting fire to any place in the world and he plans on using this to force the Clankers to surrender.
Alek and Deryn turn on the metal detecting device and they notice that it is now detecting some metal in Mr. Tesla's room. Deryn sneaks into the inventor's room to search for anything large and metal he might have that's activating their metal detector device. While hiding under Mr. Tesla's bed, she finds something metallic and takes a sample of it for Dr. Barlow who sends it to a specialist for analysis. She thinks that Mr. Tesla is insisting that the Leviathan go to New York because he plans on throwing a big show when he activates the Goliath.
Alek reads a newspaper about Deryn's father and figures out that Deryn is a girl. He doesn't know how to handle this information but he feels betrayed because he has told Deryn the truth about his identity and she has been lying to him all this time. After trying to ignore it for some time, he finally confronts Deryn about her true identity as a girl. The two get into a fight where Alek figures out that Deryn is in love with him. He's uncomfortable with the change in their relationship, now that he knows Deryn is a girl so he avoids her.
The Leviathan next heads to Tsingtao, a Clanker port on the Chinese mainland. The dignitaries on board the Leviathan meet with the British Ambassador who has set up a stage for Mr. Tesla to regale everyone with his latest inventions. While in Tsingtao, Alek and Deryn make up and they explore the city together. After finishing their business in Tsingtao, the Leviathan heads to New York, on the insistence of Mr. Tesla. As they cross the pacific they are caught in a dangerous storm. Alek and Deryn end up being trapped on the hull of the Leviathan while the storm rages. Alek hits his head and Deryn kisses him because she is so overwhelmed by adrenaline and emotions. After kissing him, she tells him that she understands their relationship won't work because he's a Prince and she's a commoner. After nursing Alek's wounds, he is still too dazed to really process what happened and Deryn is too embarrassed to bring it up again.
The storm blows the Leviathan across the Pacific to California where they are greeted by a Mr. Hearst who wants to capture “moving-pictures” of the Leviathan's arrival in New York. Deryn is asked to guide the film-maker and his men around the ship and she soon gets annoyed at their nosiness. One of the news reporters turns out to be Eddie Malone, an irritating man who stalked Alek and Deryn in the second book of the series. Eddie Malone sneaked onto the Leviathan because he works for the New York World which is a rival newspaper company from the New York Journal which is owned by Mr. Hearst. Dr. Barlow informs Alek and Deryn that Mr. Hearst is actually a German immigrant and that his purpose in filming the Leviathan will likely be to create propaganda against the British and Darwinism. Later, Mr. Hearst invites a few more of his people for dinner aboard the Leviathan, one of them is a woman named Miss Rogers who claims to be a girl reporter. Miss Rogers, Eddie Malone, Deryn, Alek, Dr. Barlow and Mr. Tesla discover that one of Mr. Hearst's guests, a man named Mr. Francis has for some reason smuggled aboard crates full of film and sugar.
As they are travelling to in California, the Leviathan is ambushed by Mexican manta ships (manta ray ships). Coincidentally, the Leviathan soon begins experiencing problems with its fuel. They find out that Mr. Hearst has tampered with the fuel (likely using the weird sugar brought in by Mr. Francis) and the tampered fuel is damaging the engines. The Mexicans seems to expect the Leviathan to be in trouble as they offer the crew some help. Later, they find out that General Villa (the leader of the small group of Mexicans soldiers) and Mr. Hearst have worked together on many movies. To make sure General Villa doesn't have any sinister reason for wanting to help the Leviathan, Deryn takes some scout wings and flies over General Villa's settlement to investigate. She notices some Clanker contraption that look like they're loaded with guns. Upon getting closer, she sees its really a movie camera mounted on a Walker and figures out that General Villa is just interested in filming the Leviathan. When Deryn tries to land, she loses control and ends up breaking her leg.
The General Villa arrives on the scene and offers to provide medical aid for Deryn and also fix the Leviathan's ship engines. General Villa's doctor finds out that Deryn is a girl. Deryn is depressed as she thinks the doctor will spread the news and she may be kicked out of the British Air Service. Later they find out that Eddie Malone has been snooping around and has figured out Deryn's secret. Deryn tells Dr. Barlow the secret and the scientist asks her if she might serve the British Empire as her personal assistant, since she's rather good at diplomatic relations and she's familiar with Dr. Barlow's creatures. Deryn thinks it might be a good idea.
After it is repaired, the Leviathan travels to New York. Once in New York, there is another assassination attempt on Alek as someone sets off a rocket on a nearby building. Alek survives and he also saves Eddie Malone's life. He decides to trade his secret identity – that of the heir of the Austrian throne – to Eddie Malone in exchange that the news reporter doesn't report about Deryn's real identity. Deryn is immensely grateful but this news is bittersweet as, now that her identity as a girl is safe, she must still fulfill her role as an airshipman on the Leviathan. She will be leaving Alek behind as the Leviathan continues on its missions.
As Deryn and Alek say good-bye, they are met by Lilit and Kizlar Agha, two of Deryn and Alek's friends. In the second book of the series (Behemoth), Deryn and Alek had helped Lilit and Kizlar Agha in launching their revolution against the Ottoman Empire while they were in Istanbul. They tell Deryn that they are now ambassadors of the Ottoman Republic. Lilit tells Deryn that the rocket attack on Alek was actually sent by the Germans to assassinate Mr. Tesla, because they fear the Goliath. She also tells them the Germans have some sort of amphibious U-boat and they plan on invading by stealth and destroy the Goliath secretly.
That evening, as expected, Deryn and the crew of the Leviathan notice a disturbance underwater that could only be the German U-boat walkers. Deryn is confused as to why the Leviathan isn't attacking, as she's worried that the U-boat walkers will get too close to Alek and his life might be in danger. Dr. Barlow reveals that, under the order of Lord Churchill, the Leviathan is not to attack until the U-boat has made its appearance on American soil. Deryn realizes that the entire plan is to have an excuse to drag the Americans into the war. She sends an urgent message to Alek to warn him.
Meanwhile, Alek is attending a dinner with all the dignitaries on Long Island. Upon receiving Deryn's message he announces that everyone should be prepared for a German attack. Mr. Tesla is confident he can use Goliath to defend himself. To Alek's horror, Mr. Tesla prepares Goliath while Leviathan is close by trying to fend off attacks from the Germans. Mr. Tesla is upset that the Germans will destroy Goliath, a machine he has invested thousands of dollars and many years of research into. Alek won't let Mr. Tesla fire Goliath also because it will send a bomb to Berlin without allowing time for people to evacuate. With the help of Count Volger, they are able to cause Mr. Tesla's machine to malfunction by using smoke bombs. Mr. Tesla continues to try to frantically power up his machine but Alek grabs Mr. Tesla's electrical stick and zaps him.
Unfortunately, Mr. Tesla already powered up Goliath and it's wild energy is causing the nearby German U-boat walkers to malfunction as well. The Leviathan also looks like it's glowing and its engines have stopped working. Suddenly another U-boat walker arrives on the scene but this time it gets tangled in the metal surrounding Goliath. Goliath unleashes the entirety of its charge on the walker, causing an explosion. Alek is glad that Deryn and the Leviathan is okay but he is shocked that he had killed Mr. Tesla.
A few days after the explosion, Dr. Barlow finally receives a reply from the specialist that was investigating the sample of metal from the metallic component Mr. Tesla had been hiding under his bed. The specialist reveals that the metal was nothing more than a sliver of a meteorite. Dr. Barlow thinks that Mr. Tesla was nothing more than a fraud and a madman and that Goliath never would have worked. For now, the war is ended because the Germans still believe that Mr. Tesla's invention is capable of doing damage.
The story ends with Deryn and Alek professing their love for each other and determining to find a way to make their relationship work, despite the class differnece.
Best part of story, including ending:
I like all the interesting creatures and machines like the Leviathan and the Walkers that Scott Westerfeld invents for his Darwinists and Clankers to use. It's fun to read and makes the story very easy to immerse oneself into.
Best scene in story:
My favorite scene was when Deryn and Alek are caught on the Leviathan's hull during a storm and Alek hits his head. I thought it was a very appropriate scene for Deryn to express her love of Alek - by saving his life! I also thought it was funny that Deryn had to kiss Alek to keep him awake because she was afraid that if he got knocked out he might not wake up again.
Opinion about the main character:
I like that Alek has matured from a slightly spoiled Princeling to a more mature and capable leader by the end of the series. I like that he sacrificed his own anonymity to the press in order to keep Deryn's true identity a secret so that she can continue to pursue her dreams aboard the Leviathan.