Ignatius "Ig" Perrish is the prime

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 and lawyer Lee seems to believe him. He stays with his parents and brother Terry, hiding from the press. After a vigil led by Merrin's father, who believes Ig to be guilty, Ig drinks heavily and wakes up with a pair of horns protruding from his head. While at his doctor to try and remove his horns, Ig, under anesthesia, dreams of his childhood, when he first met Merrin after the death of her mother. A young Ig and his friends play with cherry bombs near the docks; Ig risks a dangerous dare to win a cherry bomb, and nearly drowns, but is saved by Lee. He later trades the cherry bomb with Lee in exchange for fixing Merrin's broken necklace; however, Lee loses two fingers when the cherry bomb goes off accidentally. Ig and Merrin bond over the fixed necklace and fall in love, frequenting a treehouse in the woods together.

The horns are revealed to force people around Ig to tell him their darkest secrets and desires. Ig goes to visit his parents, but under the power of the horns his mother reveals that she doesn't want him as her son, while his father tells him that he is worthless without Merrin. Ig goes to a bar, where he goads the reporters into a brawl and attempts to find evidence that he didn't kill Merrin. Ig gets three people to confess their darkest secrets, leading the bartender to burn down his bar. From one of his confessions he also learns that the new witness for the prosecution is from the diner where Merrin had broken up with him the night she died. Ig finds the waitress, and discovers that she has been fabricating her stories to become famous. When Ig talks to Terry, he learns that Terry had driven Merrin from the diner the night she died. Touching Terry's skin, Ig sees what happened that night: Merrin left the car en route to her home and ran into the woods; Terry passed out in the car, and woke up the next morning with a bloody rock in his hand before finding Merrin dead under the treehouse. Afraid that he would be implicated, he threw the rock away and fled. Infuriated, Ig brutally assaults Terry until he is arrested by a police officer, another childhood friend named Eric. The next morning Ig is released from jail with Lee's help, discovering that Lee had been wearing Merrin's cross necklace and as a result couldn't see Ig's horns.

Ig realizes that snakes are following him wherever he goes, and uses them to exact vengeance against the waitress. He also forces Terry to overdose on drugs, causing Terry to be tormented with memories of Merrin's death. Ig meets Lee by the docks and pulls off Merrin's necklace; exposed to the horns, Lee falls under their influence, admitting he killed Merrin. In flashbacks, it is shown that Lee was also in love with Merrin and was deeply jealous of Ig throughout their childhood. Lee followed Merrin into the woods, thinking that she had broken up with Ig to be with him, and in a jealous rage he raped her, killed her with a rock—which he planted on Terry—and stole her necklace. As Ig sees all of this, Lee overpowers him and lights him on fire in his car, causing Ig to drive into the bay. To the authorities, Lee claims that Ig confessed to the crime and committed suicide. In reality, with the power of the horns, Ig survives, horrifically burned and disfigured.

Merrin's father, who now believes Ig's innocence, gives Ig the key to Merrin's lock box. When Ig puts on Merrin's cross, his body is restored and his horns disappear. In the box, he finds a note from Merrin that explains that she knew he was going to propose and wanted to accept, but she was dying from cancer and didn't want him to suffer through what her father did, so she pushed him away under the pretense of loving someone else. Ig confronts Lee, who does not remember their earlier fight, and leads him into the woods where Merrin was