The plot concerns a young man who is accused of murdering his girlfriend. One morning he begins growing horns that seem to give him the powers of Satan.
This film includes the following tropes:
- Adaptational Heroism: Several characters come across as more sympathetic than their book counterparts - notably Terry who, in the book, was aware of the identity of Merrin's killer and knowingly concealed it and Lee Tourneau, who's nowhere near the villain level of Lee in the book.
- Aerith and Bob: The Perrish brothers; Ignatius and... Terry.
- Anti-Hero: Ig.
- Armoured Closet Gay: Ig's former friend Eric, who was a homophobe even in childhood, is revealed to be gay and mutually attracted to the also-closeted cop he's partnered with.
- Asshole Victim:
- The waitress who lied about seeing Ig drag Merrin into his car the night she died to get on television in hopes it will be a springboard into stardom is the first person he goes after when he embraces the horns, ruining her looks by sending a den of snakes to bite her face repeatedly.
- Also Lee, who raped and murdered Merrin in a fit of jealous rage and when he confesses to the crime he then gleefully murders Eric, injures Terry and mortally wounds Ig. He meets his end when the fully empowered Ig impales him with his horns, then sends a snake to slither though the hole and force its way down his throat.
- Atomic F-Bomb: Ig lets out three of these in a row after Lee lies that Merrin left him because they were in love with each other, each one longer than the last.
- Bad Powers, Good People: Ig grows horns, controls snakes, and literally brings out the worst in people, but he uses these to find and punish Merrin's killer.
- Bedmate Reveal: Ig wakes up with Glenna after she drags him away from Merrin's memorial.
- Big Brother Instinct: Part of Terry's drug usage after Merrin's death is spurred by his lack of this - namely, numbing the guilt he feels for not coming to Ig's defense because what he knows implicates him instead. By the end of the movie, he's this in full force, to the point of taking a shotgun to the leg and holding Ig's demonic form as he dies.
- Big Red Devil: Ig's devilish appearance gets an upgrade after he's set on fire, with most of his skin burned red and black.
- Bittersweet Ending: Ig is reunited with Merrin's spirit in some sort of afterlife, after clearing his own name.
- Black Is Bigger in Bed: A female patient waiting to see the doctor along with Ig is the first person to blurt out any dirty secret reveals she's sleeping with her black golf trainer and adds gleefully that he's well-endowed. He briefly sees her memory of her going at it with him very pleasurably.
- Book Ends: The film begins and ends with the same scene of Ig and Merrin together.
- Brick Joke: When heading to his usual bar, Ig is hounded by reporters desperate for a big break, so he tells them that he'll grant an exclusive interview with whichever reporter manages to beat the shit out of the others. They start doing so with gusto, allowing him to enter the bar freely. When he comes out and gets into his car, the last reporter standing quietly says "But... my interview."
- Devil's Pitchfork: Ig carries around a pitchfork for a good portion of the film, aiding to his devil ensemble.
- Drowning My Sorrows: Ig has become quite the alcoholic since Merrin's death, and his getting blackout drunk that night is part of why he questions if he did kill her. And while his brother is The Stoner, he's gotten much worse in his usage because he was the last person to see Merrin alive - and is convinced he was the one to do it, also while blackout drunk.
- Entitled to Have You: Lee felt this way towards Merrin and even thought she broke up with Ig to be with him. Merrin adamantly repeating that she only loves Ig is what causes him snap and murder her.
- Fiery Cover-Up: The lab with the physical evidence from the murder is burnt down. It's later shown that his father called in a favor to do it, believing the evidence would implicate his son.
- Foreshadowing:
- In the first scene, when Ig promises he'll love Merrin for as long as he lives, she tells him just to love her for as long as she lives.
- The fact that Lee is the only one who "believes" Ig didn't murder Merrin is a bit of a red flag.
- Glass Cannon: Ig is this, at least in the film. Even when he's in full-on demon mode, summoning snakes, big horns and Super-Strength, and despite being super hard to kill, things seem to cause as much pain and damage to Ig as they would any normal human. Lee nearly beats him to death in their first altercation, and mortally wounds him in the second.
- God Is Flawed: Ig resents that with all her faith, and with all her goodness, God did not protect Merrin from dying horribly.
- Good Is Not Nice: Despite being more of an anti-hero, Ig is by no means bad. His methods of interrogation are often morally questionable and unpleasant, but it's all for the sake of solving his girlfriend's murder.
- Green-Eyed Monster: Lee is resentful of Ig for winning Merrin's heart, which he openly admits during their scuffle.
- Held Gaze: Merrin first catches Ig's attention in church by flashing light in his face so he'll look up and see her staring at him.
- I Love the Dead: Although it doesn't actually happen, the lying waitress makes her story more salacious by claiming that Ig had anal intercourse with Merrin after killing her.
- I Want My Beloved to Be Happy:
- Ig eventually finds a letter from Merrin explaining their breakup. She's dying of the same kind of cancer that killed her mother, and decided to push him away to spare him the pain of watching her die slowly and horribly. She hopes he'd find another girl and move on before realizing the truth..
- After Glenna admits to Ig that she's always loved him but knew he would never love her back, he gently tells her that she should go to New York to find the right person for her.
- Love Triangle: Lee and Ig both love Merrin, but she only has eyes for Ig. This becomes a major plot point. Glenna, Merrin and Ig also counts.
- Meaningful Name: Ignatius literally means "fiery one."
- Memento MacGuffin: Merrin's cross, which belonged to her mother first. It's the first clue that Lee had killed Merrin, and it also keeps him from seeing Ig's horns. Later, it lets Ig return to his human form after he'd suffered burns that made him look like a Big Red Devil.
- A Mind Is a Terrible Thing to Read: Ig's horns cause others to blurt out their innermost thoughts with little provocation - he's really hurt by what his parents have to say.
- Missing Mom: Merrin's mother died of cancer in her childhood, which she doesn't like to talk about. This becomes relevant later-it turns out to be hereditary, so she gets the same kind.
- Punished with Ugly: Ig decides to punish the lying waitress (one of the few witnesses to his last interaction with Merrin) by having snakes repeatedly bite her, damaging her vanity and taking away her chance at fame from being a star witness in his murder trial.
- Red Right Hand: Lee Tourneau, the villain of the film, lacks two fingers on one hand.
- Screw the Rules, I Have Money!: Everybody believes Ig killed Merrin and that his family's money saved him from justice. Including his family, who paid to "save" him from justice.
- Secret Test of Character: Potentially invoked in Ig's first post-horns encounter with Merrin's father; while the man says that he wants to shoot Ig for killing Merrin, by that point Ig has gained enough experience of the horns to realise that, if the other man actually did want to shoot Ig, he would have done it already.
- Secretly Dying: Merrin had late-stage breast cancer when she was murdered. She only left Ig to spare him the trauma of watching her die.
- Talking Is a Free Action: At least two characters spill their guts to Ig while he's actively assaulting or trying to kill them.
- Touch Telepathy: One of Ig's abilities is being able to peer into someone's memories if he's touching them.
- Troubling Unchildlike Behavior: The little girl at the doctor's office spends most of her screen time screaming and smashing the waiting room toys, then tells Ig she hates her mother wants to burn her alive with matches while she's sleeping.
- Vengeance Feels Empty: Merrin's father prayed every day for Ig to die when it's believed that he killed her. When Ig was believed to have committed suicide, he states that it didn't help one bit.
- The Unfavorite: Ig's father admits under the influence of the horns that he's always been closer to Ig's brother, because they share a passion for music. Both parents admit that, with Ig's murder trial, they both wish he'd just go away.
- Unlucky Childhood Friend:
- Glenna to Ig. She admits to him that she knew she could never compare to Merrin.
- Also Lee to Merrin, which ultimately ends in her death.
- Wall Bang Her: Ig has sex with Merrin against a tree.
