The film had its world premiere at the 2013 Toronto International Film Festival, and was released theatrically in the United States on October 31, 2014.
Upon its release, the film received mixed reviews from critics, praising the originality and Daniel Radcliffe's performance but criticizing the script.
Ignatius "Ig" Perrish is the prime suspect when his girlfriend Merrin is raped and murdered. Despite his declarations of innocence, he is shunned by the community, and only his childhood friend Lee seems to completely believe him. Even his parents and his brother Terry seem pessimistic about the situation. After a nighttime vigil led by Merrin's father, who believes Ig to be guilty, Ig drinks heavily and is taken home by another childhood friend of his — Glenna — only to wake up the next morning with a pair of horns protruding from his head. He soon learns that the horns force people to reveal their darkest inner-desires to him and they seek his permission to enact them. While at his doctor to try and remove his horns, Ig, under anaesthesia, dreams of his childhood where he first met Merrin after the death of her mother from cancer. After Merrin uses a cross necklace in church to get his attention, Ig finds it broken in a pew and has his friend Lee repair it so he can return it to her. In exchange, Lee takes a cherry bomb Ig got earlier for a dangerous stunt he performed and from which Lee saved him from drowning.
He decides to test his horns on his parents, learning the disheartening truth from them that they are disappointed in him for not being as talented as Terry, who is a professional musician. But Lee seems unaffected by the horns, and is unable to see them. Learning that a new witness for the prosecution is the waitress who witnessed Merrin break up with him on the night she was killed, Ig uses his horns to learn she had been embellishing her stories in order to become famous. He approaches Terry to reveal this to him, and learns from him that he drove her home that night, something he hadn't told police. Touching his skin, Ig sees what happened that night: Merrin had left his car enroute and Terry passed out, awakening the next morning with a bloodied rock in his hand and discovering her dead body, though he does not remember killing her. This enrages Ig, who brutally beats Terry up until another friend from their childhood — Eric, now a police officer — arrests him. Ig discovers snakes follow him wherever he goes, and embraces his abilities, using them to exact vengeance against the waitress, wounding her so that her vanity is no longer a reason for her to continue to lie about what she saw that night. He forces Terry to go on a drug binge to relive the night of Merrin's death, for not admitting his side of the story in order to save his own skin. Ig has Lee meet him at the docks, and notices that he is wearing Merrin's cross. After he removes it, Lee suddenly sees the horns that it had protected him from and falls under their influence, admitting he killed Merrin. Ig uses his abilities to see that Lee was enraged when Merrin still loved Ig, and Lee thought she broke up with him in order to be with him. Lee then raped Merrin, struck her and killed her with a rock — which he planted on Terry — and stole Merrin's necklace. Weakened by his anger, Ig is overpowered by Lee, who brutally beats him, then lights him on fire in his car. Ig drives off into the bay and supposedly drowns. Lee reports to the news that Ig confessed to him before committing suicide, but Ig survives his injuries due to his powers.
After meeting with Merrin's father, who no longer believes him to be responsible for her death, he gives Ig a key to a lock box in their treehouse, and allows Ig to keep her necklace when Ig offers it to him. When he puts it on, his body is restored and his horns are removed. He goes to the treehouse and finds a note from her that explains that she knew he was going to propose, and that she wanted to be his wife, which makes Ig happy. However, she was suffering from the cancer that had killed her mother and she couldn't put him through that, so she made a pretense of leaving to be with someone else in order to push him away. Ig confronts Lee, who does not remember their earlier confrontation due to the horns' influence and he leads him into the woods. Lee confesses again to the murder, and Eric appears with Terry to arrest him. At first cooperating, Lee turns on Eric, killing him and severely injuring Terry. As he prepares to kill Ig, Ig tears off the necklace, and sprouts a pair of angel wings, which cause him to take flight. The wings are soon engulfed by a fire, that also consumes Ig's body, turning him into a demonic creature who uses his preternatural strength and his abilities to control snakes to mercilessly kill Lee. With his quest for vengeance done, Ig dies from his injuries, claiming that his vengeance was all-consuming. In the afterlife, however, Ig is reunited with Merrin.