During the final days of World War II, the Nazis, on the brink of defeat, resort to black magic to turn the tide of the war. With the help of Russian mystic Grigori Rasputin, they conduct a ritual on a remote Scottish island to summon forces that would bring about victory. However, the Allies, led by Professor Trevor "Broom" Bruttenholm, intervene, and Rasputin is seemingly killed during the process. Instead of demonic entities, the ritual brings forth a red-skinned, infant demon with a large stone right hand. The Allies adopt the creature, naming him Hellboy, and Broom raises him to fight against the dark forces threatening humanity.

Hellboy grows into a formidable but grounded paranormal investigator, working for the Bureau for Paranormal Research and Defense (BPRD) alongside the aquatic empath Abe Sapien and Liz Sherman, a woman with pyrokinetic abilities and a complicated relationship with Hellboy. As they face a variety of supernatural threats, the team uncovers a plot that signifies Rasputin's return and his plans to awaken an ancient, god-like creature, the Ogdru Jahad, to trigger the apocalypse. Hellboy must come to terms with his own nature and destiny, grappling with his purpose as an agent of good who is, in essence, born from an inherent darkness.