The movie follows the titular character, Hellboy, as he is sent on a mission to England to defeat Nimue, the Blood Queen. Nimue is a powerful sorceress who was once Merlin's consort and seeks to destroy humanity in revenge for what she perceives as their mistreatment of witches.
Alongside his allies, the psychic Alice Monaghan and the Were-Jaguar Ben Daimio, Hellboy faces off against Nimue and her army of monsters. Eventually, he learns that Nimue's ultimate goal is to unleash the apocalypse and bring about the end of the world.
Meanwhile, Hellboy also grapples with his own identity as a demon who has been raised by humans and serves the Bureau of Paranormal Research and Defense (BPRD). He must confront his own role in the world as the son of the demon Azzael and the powerful mage Sarah Hughes.
As the battle with Nimue intensifies, Hellboy must make a choice between fulfilling his duty to protect humanity or embracing his true nature and joining forces with Nimue to bring about the apocalypse. In the end, he chooses to do what is right, even at great personal cost, and saves the world from destruction.