It employs the third-person omniscient, nonlinear narrative in telling the story of Ig Perrish, who—in the aftermath of his girlfriend Merrin Williams' mysterious rape and murder—awakes one morning to find horns growing from his head and diabolical powers at his command.
After a drunken night in the woods containing an old foundry, near where his girlfriend's corpse was discovered, twenty-six-year-old Ignatius "Ig" Perrish wakes up one morning to find that he has sprouted bony, sensitive horns from his temples.
Ig is the second son of a renowned musician and the younger brother of a rising late-night TV star, Terry Perrish.
Ig had position and security within his hometown of Gideon, New Hampshire, but the rape and murder of his girlfriend, Merrin Williams, changed all that.
Throughout the day he makes people give in to the ugly urges: including two police officers, his church's priest and a nun, and others.
Back in his high school years, Ig becomes infatuated with Merrin at church when she flirtatiously reflects light off her cross necklace into his eyes.
There, Eric Hannity bets that if anyone rides naked down a perilous trail in the woods, he will give them a cherry bomb that he and Terry have been playing with over the summer.
Merrin explains that Ig, who is about to go to England for six months for his job, should openly pursue other women while there, in order to gain some more romantic experience.
Returning to his parents' home, Ig touches a sleeping Terry's wrist and suddenly sees the events of the night of Merrin's murder through his eyes.
Terry was riding in Lee's car when they pick up Merrin, but is drunk and high and passes out while the actual murder takes place.
Lee tosses Ig into the latter's AMC Gremlin, douses the car with gasoline, and lights it on fire before it crashes into the river.
He consistently finds more meaning than is intended from Merrin's gestures and choice of words, believing her to be sexually interested in him and, knowing that Ig will soon be leaving for abroad, eager to begin an affair with her.
Exploring the attic, Ig finds a group of papers written in Morse code and a mammogram that reveals that Merrin too had breast cancer.
Returning to the foundry Ig deciphers the Morse code in which Merrin, knowing she will die from breast cancer, encourages him to find another romantic partner.
He uses her cellphone to call Lee and, mimicking Glenna's voice, persuades him to drive to the foundry, where Ig hopes to ambush and kill him.
As Terry goes to use Glenna's phone to call emergency services, he is bitten by a venomous snake that Ig had placed there to attack Lee.
Approximately a decade ago, Hill wrote an epic fantasy novel entitled The Fear Tree... that involved a character with the ability to divine people's most closely guarded secrets.
This concept was reworked for a novel Hill wrote entitled The Surrealist's Glass, wherein the protagonist acquired a magical lens which allowed him to see people's secrets.
According to Hill, "writers tend to revisit the same themes, tropes, places, and concerns, again and again, until they figure out how to use them in a satisfying way," which is what he did with this premise until he "finally got it right with Horns.