Hunter Steel, a boy aspiring to be an adventurer, learns of the existence of an unknown subterranean world called the "Inner World"—a place where no human has ever set foot—from his grandfather's diary. He travels to a South American-style pyramid, said to be the entrance to that world. Guided as if by a mysterious voice, Hunter arrives in the Inner World, where he becomes caught up in a disturbance caused by a mysterious insect army known as "Insecters"—insects that stand upright and speak human language. In this world, the Insecters harbor ambitions of world domination and expand their influence day by day. By chance, Hunter obtains the proof of the "Spider Riders," righteous warriors who fight alongside giant spiders native to the Inner World—spiders encased in robotic-like exoskeletons. At the scene, he partners with Spider Shadow (despite Shadow having absolutely no intention of doing so) and together they repel the Insecters. Later, Hunter meets a girl Spider Rider named Corona, who arrives on the scene. Learning of this world's plight from her, Hunter vows to fight as a Spider Rider to restore peace to the Inner World. However, Shadow declares, "Humans from the surface world bring nothing but calamity," and remains reluctant to join forces with Hunter.