Set in a war-torn reimagining of 1914, Leviathan follows Alek, a fugitive prince of the Austro-Hungarian Empire, and Deryn Sharp, a daring young girl disguised as a boy to serve in the British Air Service. When fate throws them together aboard the HMS Leviathan, a massive, living airship made from genetically engineered creatures, they embark on a world-changing adventure that will challenge everything they know about war, loyalty, and identity.

The alien invader Goa plots to conquer the Earth. He first warns the Murakami family (father Atsushi, mother Tomoko, and son Mamoru) of their invasion, and demonstrates his powers by transporting them to a prehistoric jungle and destroying a Giant Dinosaur before their very eyes. But they will not agree to surrender to Goa, so hope comes in the form of Magma, an armored, golden giant with long hair and antennae. He and his human-sized wife Mol, both created by the wizard Earth are sent to defend our world against Goa. They befriend Atsushi and Mamoru, the latter has Magma emotionally touched, as he wanted to have a child with his wife Mol, so Earth creates a duplicate of Mamoru, named Gam Earth gives Mamoru a whistle, with which he can call Gam Mol and Magma in times of crisis. So when Goa unleashes his various daikaiju, chances are, Magma, Mol, and Gam will fly to the rescue.

In a Kaiju-filled Japan, Kafka Hibino works in monster disposal. After reuniting with his childhood friend Mina Ashiro, a rising star in the anti-Kaiju Defense Force, he decides to pursue his abandoned dream of joining the Force... when he suddenly transforms into the powerful "Kaiju No. 8." With help from his junior colleague Reno Ichikawa, Kafka hides his identity while striving towards his life-long dream of passing the Defense Force exam and standing at Mina's side. But when mysterious intelligent Kaiju attack a Defense Force base, Kafka faces a crucial decision in a desperate situation...

Mars has been colonized and is a world where children have been replaced by robot servants known as "dolls." Layla is a skilled fighter with a tragic past who travels about the world. Her companions are Nei, a strange and unique doll with some unknown ties to Layla, and Speedy, who is a doll breeder. The founders of Mars see the trio as a threat to their world, and each time they attack Layla and Nei a bit more of their mysterious past and future is revealed.