The setting of Leviathan takes place in a dark fantasy world mixed with cyberpunk and alien technologies. The country is ravaged by a disease called Decade, which is caused by the Plague King. The "Decade" afflicts the land at irregular intervals, and lasts for 10 years. Oliver Vertran, the protagonist and main character, was a young boy when the Decade began. When his mother is murdered, he becomes the adopted son of her murderer. Oliver and his friends Kael and Darina, with the help of a Vargoille called Edna, seek to punish a guardian for killing his mother.