A professor who studies folklore opens a forbidden book and is possessed by the ancient Japanese god of thunder and lightning, who is determined to return and rule the world. The Superpower Research and Defense Bureau sends Hellboy and a group of agents to investigate the case. During the investigation, when Hellboy picks up a samurai sword, he enters a world full of Japanese legends, a world full of various ghosts and monsters. Agents Kate Corrigan and Russell Thorne begin tracking the controlled professor to find and bring Hellboy back to the human world.