In the year 1795, Dr. Victor Frankenstein (Aden Young) creates a monster (Aaron Eckhart), a being assembled from deceased bodies but devoid of a soul. Upon realizing the horrors of his creation, Victor rejects it and attempts to destroy it. Enraged, the creature retaliates by killing Victor's beloved wife, Elizabeth (Virginie Le Brun). This sets Victor on a relentless pursuit to confront the monster in the Arctic, but he ultimately succumbs to the brutal elements.
After burying his creator, the creature finds himself under siege by demons. Fortunately, he is saved by gargoyles Ophir (Mahesh Jadu) and Keziah (Caitlin Stasey), who bring him before their queen, Lenore (Miranda Otto), and the commander Gideon (Jai Courtney). Lenore reveals that they were formed by the Archangel Michael to wage war against demons on Earth and safeguard humanity. Although they offer the creature the name "Adam" and an invitation to join their ranks, he chooses to decline, taking instead weapons that can destroy demons and trap their souls in Hell.
As centuries pass, Adam continues to fend off relentless demonic threats. In a modern-day altercation at a nightclub, a tragic incident leads to the death of an innocent police officer. The gargoyles summon Adam once again to account for his actions. Meanwhile, the demon Helek (Steve Mouzakis) informs his master, the demon-prince Naberius (Bill Nighy), who is masquerading as billionaire Charles Wessex, of Adam's continued existence. Wessex enlists scientist Terra Wade (Yvonne Strahovski) to brainstorm ways to reanimate corpses.
Terra’s work is making strides in reviving rats; however, she needs Frankenstein’s research to break into human revival. Naberius, eager to accelerate their experiments, dispatches a legion of demons, led by Zuriel (Socratis Otto), to capture Adam and acquire the knowledge to give life once more. Before Lenore can punish Adam for the officer’s death, their cathedral is invaded by demons. In the chaos, Adam persuades Ophir to free him.
The subsequent battle is devastating, with many gargoyles, including both Ophir and Keziah, being “ascended” (essentially trapped in Heaven), leaving Lenore at the mercy of the demons. Adam and Gideon race to rescue her, leading to Gideon's exchange of Lenore for Victor's journal, which holds the key to the reanimation secrets. Following Zuriel to the Wessex Institute, Adam discovers that Naberius intends to replicate Frankenstein’s experiment, aiming to create an army of reanimated corpses that can house the souls of the fallen demons.
After a successful retrieval of the journal, Adam escapes but soon confronts Terra, revealing Naberius's dark plans before they are attacked by Zuriel, whom Adam manages to defeat. Forewarning the gargoyles of Naberius's intentions, Lenore sends Gideon to eliminate him and reclaim the journal. A fierce confrontation ensues, forcing Adam to "ascend" Gideon, then resolve to burn the journal, eliminating its secrets before the gargoyles can pursue him.
Eluding them, Adam leads the gargoyles to the Wessex Institute, culminating in a fierce clash with Naberius's forces, where he successfully takes down Dekar (Kevin Grevioux). Amid the ensuing chaos, Adam ventures into the Institute to save Terra, who has fallen into Naberius's clutches. After Naberius showcases his true monstrous form and activates a reanimation machine, he attempts to seize Adam’s body. However, Adam withstands this attack, having nurtured his own soul.
As the gargoyles try to subdue the reanimated demons, Adam etches the symbol of the Gargoyle Order onto Naberius, sanctifying his essence and unleashing his complete destruction, sending him and all the demons in his cohort to descend. The cataclysm collapses the entire building. Commending Adam’s valor, Lenore saves him and Terra, forgiving him for Gideon’s fate. With his weapons reclaimed, Adam parts ways with Terra and embarks on a quest to protect humanity while hunting demons for the sake of others, thereby embracing his destiny and his true identity as "Frankenstein."
