Plot

A prince of the mythical world starts a rebellion against humanity in order to rule the Earth. Hellboy and his team must fight to stop him from locating the all-powerful Golden Army.

After saving the world in Hellboy (2004), humankind's strange protector Hellboy now struggles to cope with a challenging reality. And as if that weren't enough, the emergence of Elven Prince Nuada only spells trouble. Now, the powerful being is bent on retrieving the fragments of a mystical crown that commands the indestructible Golden Army: an omnipotent legion of clockwork warriors designed to seek and destroy humans. Before this unstoppable new menace, all hope rests on Hellboy and his squad of supernatural defenders. But they are no match for Nuada's invincible forces. Has Hellboy and his justice team bitten off more than they could chew?—Nick Riganas

In this continuation to the adventure of the demon superhero, an evil elf breaks an ancient pact between humans and creatures, as he declares war against humanity. He is on a mission to release The Golden Army, a deadly group of fighting machines that can destroy the human race. As Hell on Earth is ready to erupt, Hellboy and his crew set out to defeat the evil Prince before The Golden Army can destroy humanity's existence.—ctfabian

In Christmas of 1955, a young Hellboy is told a bedtime story by his adoptive father, Trevor Broom, involving an ancient war between man and magical creatures, started by man's greed. After years of battle and defeat of the magical creatures' forces, the master of the goblin blacksmiths offers to build an indestructible golden mechanical army for King Balor, the one-armed king of the elves. Influenced by his son Prince Nuada, Balor orders the building of this Golden Army. A magical crown is also created to control the Golden Army, by someone of Royal blood, if unchallenged to the right to control the army.

Soon the humans attack again but the Golden Army devastates them. This again saddens Balor as he finds the Golden Army to be remorseless. Balor creates a truce with the humans to stop the bloodshed: Man will keep his cities and the magical creatures will keep their forests. Nuada does not agree with the truce, and leaves in exile. The magical crown controlling the army is broken into three pieces, one piece going to the humans and the other two kept by the elves, so the Golden Army can never be used again.

In the present, Nuada declares war on the humans. He collects the first piece of the crown from an auction, massacring everyone at the site by unleashing tooth fairies, and kills his father for the second piece of the crown. His twin sister, Princess Nuala, escapes with the final piece. Meanwhile, Hellboy is having issues with his girlfriend Liz, and trouble accepting that their organization, the B.P.R.D., must remain undercover. While investigating the massacre at the auction, Hellboy allows himself to be revealed to the world. In the commotion, Abe Sapien discovers that Liz is pregnant. Furious about the public unveiling, the Bureau's superiors send a new B.P.R.D. agent, the Ectoplasmic medium Johann Krauss, to take command. With Krauss in charge, the team tracks the tooth fairies to the troll market, an enormous merchant city hidden under the Brooklyn Bridge, to search for clues.

Krauss has the ability to eject an Ectoplasmic substance, which once inhaled by the victim, allows Krauss to control any being living or mechanical. Abe stumbles onto Nuala inside the troll market, who has obtained a map leading to the Golden Army, and quickly falls in love with her. She is brought under B.P.R.D. protection following an attack by Nuada's sidekick, the troll Wink, and an elemental forest god, both of which Hellboy kills. During the elemental fight, Hellboy is questioned by Nuada whether it is right to fight for the humans when he too is considered a monster.

Nuada tracks his sister to B.P.R.D. headquarters using their magical bond, which causes them to share wounds and read each other's thoughts to an extent. Nuala, sensing her brother's arrival, throws the map, along with the canister in which it came, into a fire and hides the final piece of the crown within one of Abe's books. However, the real map is on the canister. Nuada kidnaps his sister and critically wounds Hellboy with his spear. Nuada tells Abe that to save Hellboy & to see Nuala again, Abe will have to bring the crown piece to him.

Unable to remove the spear shard, Liz, Abe and Krauss take Hellboy to the location of the Golden Army in The Giants Causeway County Antrim, Northern Ireland. They encounter a Bethmoora goblin, and he brings them before the Angel of Death, who has been awaiting their arrival. Though warned that Hellboy will doom humanity if he lives, and that she will suffer the most from it, Liz pleads for Hellboy's life. The Angel removes the shard from Hellboy's chest and tells Liz to give him a reason to live. She reveals to Hellboy that he is going to be a father, and he recovers.

The goblin leads the team to the resting place of the Golden Army, where Nuada awaits them. In exchange for Nuala, Abe gives him the last piece of the crown. With the crown reformed, Nuada awakens the Golden Army, ordering them to kill the team. The group is unable to defeat the soldiers as they magically repair themselves. Hellboy challenges Nuada for the right to the crown, and Nuada is forced to accept, since Hellboy's father was a Prince of Sheol. Hellboy defeats Nuada and spares his life, but Nuada tries stabbing him. Nuala commits suicide to stop her brother. As Nuala dies, so does her brother: the dying Nuada tells Hellboy he will have to choose whether humanity or the magical beings must die.

Abe psychically tells Nuala his feelings for her before she and her brother die. Liz melts the crown, shutting down the Golden Army. As the team leaves the underground compound, Tom Manning reprimands them. Hellboy, Liz and Abe resign from the B.P.R.D. Hellboy contemplates his future life with Liz and their baby. Liz corrects him, saying "babies", surprising him with the fact that she is pregnant with twins.