In the small city of Rockwall, Texas, a teenager named Sawyer narrates the story of how a prank on a friend led to the deaths of most of his friend group, and how he bears responsibility for it all. The novella unfolds as a first-person confession, with Sawyer's increasingly distorted reasoning revealing the gap between what he believes is happening and what is actually occurring.
Sawyer belongs to a tight-knit group of five teenage friends: himself, Shanna, Danielle, Tim, and JR. When Shanna's mother forces her to work at the main movie theater in Rockwall to pay off a landscaping debt, Shanna sneaks the others in through an emergency exit. They are caught, and the friends are banned from the theater. The consequences fall hardest on Shanna, who is paired with a more trusted coworker and subjected to random head counts.
Sawyer devises a prank to make amends and recapture their childhood spirit. Years earlier, in sixth grade, Tim found a naked white mannequin half-buried in a muddy creek behind JR's property on Rabbit Ridge in Heath, Texas. The five friends spent that summer playing with the mannequin they named Manny, dressing him in their fathers' clothes, posing him on neighbors' lawns, and staging pranks around town. When they outgrew these games, Manny ended up forgotten in Sawyer's garage.
Sawyer's plan is to smuggle Manny into the theater where Shanna works. Tim's uncle uses a band saw to split Manny's legs so each friend can hide a piece on their body or in bags. They buy tickets this time, sneak the torso in during a staged distraction, and assemble Manny during the trailers, seating him with a ticket stub in his shirt pocket. The film is the third installment of a blockbuster superhero franchise.
The prank fails in a way Sawyer cannot process. JR tips off staff that someone has sneaked in, and the assistant manager checks stubs row by row. When he reaches Manny, he shines his flashlight, nods, and moves on. Tim cannot produce his stub and is identified as one of the banned group; he and Shanna are escorted out. Sawyer stays for the rest of the film, watching Manny obsessively. When the credits roll, Manny appears to stand and file out with the crowd, legs stiff, arms swinging. Sawyer vomits into his cupholder.
On the walk home, Danielle and JR offer rational explanations. Sawyer, who has been off his medication for three days, insists he saw Manny walk out. The group splinters. Shanna cuts off contact. Tim is grounded. Sawyer constructs a theory that Manny is alive and dangerous.
When a Mack truck veers off a service road and crashes through Shanna's bedroom, killing her mother and younger sibling, the community treats the disaster as random tragedy. Sawyer interprets it as Manny's first strike. He convinces himself that Manny has consumed stolen Miracle-Gro fertilizer from neighborhood sheds and grown to enormous size, hiding in nearby Lake Ray Hubbard. He finds corroborating evidence in a torn-open bag of fertilizer in his family's shed, neighborhood flyers about stolen gardening supplies, and the fact that Shanna had been downloading the superhero movie the night she died. Sawyer later admits he lied to his friends about Manny having moved onto his father's Kawasaki motorcycle on his own; he placed the mannequin there himself and calls the lie "Miracle-Gro" for the way it helped his theory grow in their minds.
Sawyer concludes that Manny is hunting the five friends who abandoned him and that his enormous size will destroy entire families as collateral damage. He decides to kill each friend before Manny can reach them, framing this as heroism. He draws a parallel to Mary Shelley's
Frankenstein, which he read in AP English, about the consequences of abandoning one's creation.
Sawyer murders Tim first, establishing an alibi by streaming a movie on his laptop before sneaking out on the Kawasaki. Tim wakes at his gaming desk and says "Saw?", the nickname only he has used since third grade. Sawyer strangles Tim with a glow-in-the-dark edger line, whispering "I love you" as Tim dies. Tim pulls a crumpled ticket stub from his pocket during the struggle, the same one he could not find the night of the prank. To prevent the death from appearing to be a suicide, Sawyer pins Tim's body to the wall with knives. Tim's young brother Drake discovers the body.
Two days later, Sawyer calls a meeting with Danielle and JR at a dock near the lake. Danielle arrives with Steve, her new boyfriend from the yearbook team. Overcome with grief, the group swims together. When police interview Sawyer, he tells them the entire story of the Manny prank, but the detectives redirect their investigation toward the assistant manager.
Sawyer next follows Danielle and Steve from Tim's vigil to a run-down dollar theater in a mostly abandoned mall, where they watch the superhero movie. Wearing a mannequin mask, Sawyer sits behind them and strangles Steve when Danielle leaves for the lobby. Tornado sirens wail and the house lights come on. Danielle returns, sees Steve dead, and recognizes Sawyer's voice when he says "I'm sorry, D." He loops the string around her throat; the force snaps her neck. Sawyer removes his mask, kisses her cheek, and touches her lips before a moviegoer tackles him. He escapes on the Kawasaki. News reports dub the killer "Blankface."
At the double funeral for Danielle and Tim, Sawyer and JR stand at the front as the last surviving members of the group. Sawyer carries the mannequin mask in his blazer, resolved to kill JR and then himself, believing that once all five friends are gone, Manny will retreat into the lake.
Sawyer's plan to kill JR during a sleepover is derailed when JR's heavily armed father takes the boys to a drive-in theater in Fate, Texas. The same superhero movie is playing. JR's family falls asleep in the car, and the boys sneak out. The assistant manager from the Rockwall theater recognizes them and causes a confrontation. JR's father wakes, finds the boys missing, and charges into the crowd brandishing two pistols. Tornado sirens sound. A boxcar tumbles across the sky. In the chaos, Sawyer tackles JR behind the playground and strangles him with the glow string while screaming up at the sky that he is "already doing it."
A voice behind him says she knows he is the killer. Shanna is alive, wearing Sawyer's father's old bowling shirt from the costume trunk in the friends' childhood fort, where she has been living since the truck destroyed her home. The truck killed her mother, sibling, and dogs, but not Shanna herself. She has been stealing food from the neighborhood, accounting for the missing groceries. Shanna tells Sawyer the mannequin never left the theater; it was found in the break room. Manny never walked, never grew, never consumed fertilizer. Sawyer has killed his friends for nothing.
Sawyer lashes the glow string across Shanna's eyes, blinding her momentarily, and flees through the tornado's destruction, which he still attributes to Manny. He finds his father's Kawasaki among the wrecked cars, realizing his parents rode the motorcycle to the drive-in after discovering their vehicles disabled by Sawyer himself. His father appears, calling his name, but Sawyer drops the glow string at his father's feet and speeds away.
Sawyer rides to the lake and rows a stolen boat onto the water, addressing Manny in a final monologue. He carries Tim's ticket stub, reframing it as admission not to a movie but to wherever Manny will take him. He imagines them sleeping in the water until the next group of children discovers "the mannequin with the falling-apart legs and the boy mannequin beside him, his face blanked out with pleasure from all the fun that's coming" (120). He declares his love for Manny, says the sacrifices are finished, and asks for the credits to roll, implying he intends to drown himself.