 A professor of folklore opens a forbidden scroll and becomes possessed by the ancient Japanese demons of Thunder and Lightning, who seek to return and dominate our world. The Bureau for Paranormal Research and Defense sends Hellboy and a team of agents to investigate, but when Hellboy picks up a samurai sword, he literally disappears into a weird wonderland of Japanese legends, ghosts and monsters. Meanwhile, BPRD agents Kate Corrigan and Russell Thorne are on the trail of the possessed professor to bring Hellboy back.