Owen and his alcoholic mother move to Masonville, Texas, after his grandparents die and leave them a house with over 1,000 acres of land. The local high school is in a maelstrom, as 11 teenagers have committed suicide. Various media outlets descend each time another teen dies, and religious protesters troll the sidewalks, calling on students to repent.
Owen worries that his new girlfriend, Jess, is growing depressed. After school, he takes her on a ride through his family’s property, hoping to cheer her up. The two hear a sound, like a torrent of running water. They find an abandoned well that is dry.
They leave, but Jess realizes she doesn't have her phone. Owen heads back to the well, alone, to find her phone. He sees an old man at the well. The old man asks Owen if he is there to help. The man lowers a bucket into the well, and the teen is amazed when he pulls up a full bucket of water.
Although the old man tries to convince Owen that the water would help him understand the town’s suicide problem, Owen refuses to drink the water. He finds Jess’ phone and takes her home, but he can’t get the old man out of his head. Owen returns to the well and find that old man is still there.
He explains that some things have to be believed before they can be seen. He promises Owen that he will get the all the answers he wants if he drinks the water. Owen drinks and is struck with an excruciating stomach pain and a headache. He barely gets home before passing out for the night.
In the morning, Owen’s stomach still hurts, but his headache is gone. When he goes outside, he sees a bizarre sight. A woman jogger runs toward him, but she is dragging 8-foot chains behind her. The chains are attached to a shackle around her neck. Black cords sprout from her head and smack the chains.
Owen runs inside, convinced he is experiencing some kind of hallucination. When he tries later to leave the house, he is again confronted by another horrifying vision of a person in pain. He also sees the word “rage” painted on his neighbor’s fence. On his own garage he sees the word “suffer.”
Owen tries to convince his friends about what he is seeing, but they think he needs therapy. His mother thinks he is using drugs. While she sleeps, Owen decides to investigate her chains to try and decipher whether he is imagining them or not. He discovers they are extremely heavy and cold.
On an open cuff of the chain he sees a name scribbled on it. On the black cords from his mother’s head he reads the word angry. On her ceiling, victim is written. When Owen questions his mother about a name on her chains, and she becomes angry. The name was of a past abusive boyfriend.
Owen returns to the well, and although it appears dry, when he draws the bucket up, it is filled with water. He takes some to give to his science teacher to see she can find any bacteria in it that would make him sick. She promises to get him results in a few days.
Stuck back at school, Owen tries to navigate his classes and the hallways, even as he sees his classmates shackled and chained. Then he is subjected to a new horror. A giant creature, as tall as the ceiling, smelling vile and covered in sludge, enters the room. It approaches another student and attaches itself to her.
Then he begins to see more of these monsters around the school and calls them Creepers. Unable to cope with this new vision of the monsters, Owen runs home. There, he receives a text from his friend Lance, telling him that Jess has dumped him and is going to the upcoming prom with her ex-boyfriend, a boy named Dan, who used to abuse her. Owen hides in his room for the weekend, hoping that on Monday his teacher will have a scientific answer for what he is seeing.
Unfortunately, tests showed no bacteria in the well water. Although he still sees the horrible visions, he is amazed when he sees a girl without a shackle. Or cords. Or chains. He loses track of her when she goes into the girls’ locker room but remembers her name is Ray Anne. He tracks her down, and she agrees to go out with him that weekend. He calls her, and the few others like her, “Lights,” because of the glow from their feet.
Owen tries to find a way to keep the Creepers from attaching themselves to his friends, but only one thing seems to be able to stop them. He calls them Watchman. They are taller than the Creepers and radiate a blinding, crystal-white light. They also exude a beautiful fragrance and wear thin gold crowns. The Creepers separate themselves from their human hosts and flee when the Watchmen appear.
Owen knows he is seeing some kind of battle between good and evil, but he does not believe in God. The school authorities believe Owen is suffering from stress and assign him a mentor. The mentor is Dr. Bradford, the father of Dan, Jess’ new boyfriend. Dr. Bradford used to be a close friend of Owen’s grandparents. He promised to help instill their beliefs in their grandson. Owen gets a bad feeling about the doctor and vows to keep his distance.
Owen finds a confidant and helper in Ray Anne. Although she barely knows him, she believes his visions. She thinks they may be demonic and promises to try to help him. She explains that she and her family are free of the Creepers because they are Christians.
On Sunday afternoon, two friends come over to play basketball. When the talk turns to Owen’s weird visions, his friends insist he take them to the well so they can drink the water. He warns them they will get sick before they see anything and tells them to come to his house first thing in the morning so he can explain what they are seeing. But the boys never come. Owen is horrified to learn they died overnight. He lies to the police about what happened, as he does not think they will believe him.
Owen is plagued by the visions he sees, including the horrific death of one of his friends, in which he sees a huge demon drag her soul into the ground while she screams for Owen to save her. He sees the same demon and learns it is Molek — the king of demons — in his school. Molek throws several lower demons into a burning hole.
Ray Anne finds Owen shivering in fear. He tells her that Molek is plotting something awful for the school. Ray Anne thinks she would be more help to Owen if she could see the visions he does, but Owen makes her promise not to go to the well.
Later that night, when she thinks he has betrayed her, Ray Anne succumbs to the torments of several Creepers. Owen begs her to remember who she is and what she believes. When she finally starts to pray, Watchmen arrive and the demons scatter from her.
Owen and Ray Anne begin to theorize what a series of notes left by the Creepers mean. The notes all contain three letters and the numbers 523. They guess 523 could be May 23, and the letters could be initials, but they cannot fathom what the Creepers have planned for that date.
A few days later, his friend Lance, Dan and several other boys abduct Owen. They want to punish him for the death of the two other boys. Though Lance can't beat his friend, Dan pummels Owen until the old man returns and saves him. He explains that Molek has ruled the land for over a century, especially since Owen’s grandparents had been satanists who made sacrifices on the land to Molek.
Dan’s father was one of their followers. As Owen starts home, Ray Anne spots him and drives him home. She returns to the well and drinks the water. When she wakes from a pain-filled night, she can see the supernatural world that Owen sees. He is horrified to learn that he, too, has shackles. He begs her to tell him the names on his chains, and they are his mother, Lance, Dan and someone named Stephen Grayson.
Owen returns home and demands his mother tell him the identity of Stephen Grayson. She finally admits that he is Owen’s father. They married young and against his parents’ wishes. When she became pregnant, his parents convinced her that Stephen would be better off without her and the baby. They offered to pay her a large sum of money each year for 18 years for Owen’s care and education, as long as she divorced their son and never contacted him again.
But the worst news was that when she tried to contact him a year ago, she found out that he went missing on a missions trip and is presumed dead. In a fit of rage, Owen runs away. After reading an interview his father did, in which he felt his gifts gave him a calling to help those in need, Owen realizes he should be helping the people in his town to fight Molek, not running away. He returns home on May 22.
As Owen and Ray Anne approach the school on May 23, Creepers are swarming it. Ray Anne goes inside to see if she can spot what is causing the disturbance while Owen searches outside. The Creepers flock toward Dan, and Owen sees two rifles strapped to his back.
Dan enters the school and begins shooting. As students flee in panic, Owen follows Dan to try to stop him. Dan takes Jess hostage in the auditorium, and Owen races to protect her. The two boys face off. Dan kicks Owen to the ground and fires his rifle. As the police storm in, Owen discovers that Dan had intended to shoot Jess first, but Ray Anne jumped in front of the bullet. She is taken away to the hospital in critical condition.
Owen stumbles to the woods and finally prays a prayer of surrender. A Watchman comes and protects him from Molek. The Watchman confides that evil did not win at the school, but Owen may not understand how until some time in the future. After the Watchman leaves, Owen is shocked to see he is now a Light, like Ray Anne. He is thrilled to be free of his chains but knows that this is just the beginning of his adventure.