In the Spirit of Howard Philips.
In 1944, the Nazis build a dimensional portal off the coast of Scotland during World War II. With the help of Russian mystic Grigori Rasputin (Karel Roden), they intend to free the Ogdru Jahad—monstrous entities imprisoned in another dimension—to aid them in defeating the Allies. Rasputin knows that this will instead be an apocalyptic event that he believes will create a new paradise. He succeeds in opening the portal with the aid of his disciples, Ilsa von Haupstein (Bridget Hodson) and Obersturmbannführer Karl Ruprecht Kroenen (Ladislav Beran), Adolf Hitler's top assassin. A small Allied team is sent to destroy the portal, guided by young Professor Trevor Broom (John Hurt), who is well-versed in the occult. The German scientists and soldiers are killed and the portal is destroyed—in the process absorbing Rasputin—while Haupstein and Kroenen escape. The Allied team discovers an infant demon with a stone right hand has come through the portal; they affectionately dub it "Hellboy".
Sixty years later, young FBI agent John Myers (Rupert Evans) is transferred to Broom's Bureau of Paranormal Research and Defense (BPRD); he meets the adult Hellboy (Ron Perlman) and a psychic, amphibious humanoid named Abe Sapien (Doug Jones, voice by David Hyde Pierce). He learns that a third BPRD member, Liz Sherman (Selma Blair), has recently checked into a mental hospital in an attempt to protect others from her volatile pyrokinetic abilities. Despite regular visits and coaxing from Hellboy, she is determined not to return. Kroenen and Haupstein resurrect Rasputin in the mountains of Moldova. The three unleash a monstrously powerful hellhound known as Sammael (Brian Steele). Rasputin imbues Sammael with the power to reincarnate and split his essence, causing two of the creature's "eggs" to hatch and mature in seconds each time one dies. Rasputin visits Liz as she sleeps, reactivating her powers and causing the near-total destruction of the hospital. Myers convinces her to temporarily return to the bureau.
Sammael's ability to multiply becomes a problem, as Hellboy repeatedly kills it, creating dozens. During a hunt for the beast, Abe is injured while retrieving some of its eggs, and Kroenen critically wounds one of the FBI agents sent with Hellboy. Kroenen, whose mutilated body is run by mechanical parts, shuts himself down, pretending to be defeated. Kroenen's body is brought to the bureau. FBI Director Tom Manning (Jeffrey Tambor) is angered by Hellboy's recklessness. Hellboy threatens Manning just as Liz returns, almost causing her to leave. Myers takes her out for coffee and to talk. Hellboy, jealous, covertly follows them. Rasputin appears at the bureau, reanimating Kroenen before they confront Professor Broom. Rasputin offers him a vision of the future, showing Hellboy is the agent that will destroy the world. Rejecting Rasputin's vision, Broom is stabbed in the neck by Kroenen and, clutching a rosary, collapses and dies.
Manning takes over the B.P.R.D. and, with the help of Hellboy and the others, locates Rasputin's mausoleum in an old cemetery outside Moscow, Russia. A team led by Manning and Hellboy enter the mausoleum, but swiftly become separated. Hellboy and Manning find their way to Kroenen's lair, where Hellboy defeats Kroenen. Telling Manning to stay back, Hellboy reunites with Liz and Myers at Sammael's nest to defend them, but the creatures overwhelm him. In an effort to help, Liz uses her pyrokinetic powers to incinerate the army of Sammaels and all the eggs. Hellboy, Liz and Myers are rendered unconscious and captured by Rasputin. To force Hellboy to release the Ogdru Jahad, Rasputin sucks Liz's soul out of her body, telling Hellboy that Liz will come back to life only if he obeys. Hellboy, not wanting to lose Liz, awakens his true power as Anung un Rama, causing his horns to regrow. He nearly releases the Ogdru Jahad, but the injured Myers reminds him of who he is and that he has the right to choose his own path. Remembering his true self and what his father brought him up to be, Hellboy rips off his horns, returning to his former self and resealing the Ogdru Jahad. As Rasputin screams his frustration at Hellboy, Hellboy stabs him with one of his broken horns.
Rasputin has one last trick up his sleeve: he is possessed by a creature from the Ogdru Jahad. The tentacled Behemoth bursts out of Rasputin's body, grows to immense size, and kills Rasputin and Ilsa. Hellboy allows himself to be swallowed while detonating a belt of live hand grenades. The explosion tears the Behemoth apart. Liz's vital signs are gone when Hellboy returns from the fight, but he whispers into her ear, and suddenly her life is restored. When she asks how her soul was returned, Hellboy replies that he simply told the creatures from the other side the cost of taking her: "Hey, you on the other side. Let her go. Because for her I'll cross over, and then you'll be sorry." She and Hellboy kiss as she surrounds them in flame.
- Hellboy (the comic) creator Mike Mignola dedicated his first HEllboy volume to HP Lovecraft among others.
- Some of the monsters are Lovecraftian inspired the Sammael creatures have characteristics of both Nyarlathotep/Cthulhu and Elder Gods are mentioned too.
- The quote at the beginning of the film is from a book mentioned many times in the Cthulhu mythos called "De Vermis Mysteriis" (actually created by Robert Bloch)