In rural Kansas, six-month pregnant Becky Demuth and her protective brother Cal are traveling by car to San Diego. Outside an abandoned white church, they stop their car after Becky feels sudden morning sickness. While parked by the road, they hear a frightened child named Tobin calling for help from the dense field of tall grass across the street. Tobin's mother Natalie screams at him from deeper within the field, begging him to stop making noise, but Tobin keeps crying. Concerned for the child's safety, Cal steps into the grass to search for him, and Becky follows shortly after. Almost immediately, the siblings lose sight of the road and each other. Cal leaps into the air to spot Becky, and she appears just a few feet away. But when Cal jumps a second time, Becky is suddenly dozens of yards in the opposite direction. Distances and directions change constantly, as if the ground itself is shifting beneath their feet. As night falls, Becky encounters Tobin's father, Ross Humboldt. Ross acts jovial and reassuring, telling Becky that finding the way out is easy if you know how to listen to the field. Meanwhile, Cal stumbles upon young Tobin, who is holding a dead crow. Tobin tells Cal that dead things do not move in the field, and leads him to a massive, ancient black boulder standing in a clearing at the center of the maze. The monolith is carved with bizarre prehistoric symbols and hums with supernatural energy. Tobin urges Cal to touch the stone, promising it will show him the way out, but Cal hesitates. The narrative shifts to Travis McKean, Becky's ex-boyfriend and the father of her unborn child. Two months after Becky and Cal went missing, Travis tracks their abandoned car to the church. He walks into the grass looking for them, only to become trapped in the same temporal trap. Travis finds Tobin alive, and through fractured time loops, encounters Becky and Cal shortly after they entered the field. They realize that the field bends not only space, but time itself; multiple versions of themselves have entered the grass across different overlapping timelines. Ross Humboldt resurfaces, but he has touched the ancient black stone and become completely possessed by its cosmic will. Believing the stone connects all living things and holds the secrets of the universe, Ross attacks the group. He crushes his wife Natalie's head like a melon when she tries to warn them, snaps Cal's neck against the monolith, and stalks Becky through a terrifying nighttime thunderstorm. Becky collapses in exhaustion near the black stone and goes into labor. Hallucinating amidst the storm, Becky passes out as humanoid grass creatures carry her newborn baby away. When Becky wakes up, Ross feeds her morsels of meat, falsely claiming they are wild seeds, before Becky horrifyingly realizes she is eating the remains of her own child. Travis arrives and engages Ross in a desperate fight near the monolith. Ross overpowers Travis and prepares to kill him, but young Tobin grabs a sharp bone and stabs Ross in the eye. Travis breaks free and strangles Ross to death with a twist of tough grass vine. Bleeding to death from his wounds, Travis realizes that the only way to end the cycle is for someone to touch the stone willingly to understand the maze and guide the others out. Travis places his hand flat on the black boulder. A flood of cosmic knowledge surges through his mind, revealing the true nature of the field. Using his newfound understanding of the field's shifting paths, Travis grabs Tobin and pushes him through a temporary spatial rift into the interior of the white church across the road. Tobin emerges inside the empty church, looking out the front doors just as Becky and Cal pull their car up in the past. Remembering Travis's final instructions, Tobin rushes outside and shows Becky the family necklace Travis gave him. Shocked by the necklace and Tobin's desperate warnings, Becky and Cal decide not to enter the tall grass. They put Tobin in their car and drive away toward town to contact the police, finally breaking the endless loop and leaving Travis's sacrifice to stand in the field forever.

The ending of In the Tall Grass resolves the complex time loop through self-sacrifice. Throughout the story, the ancient black monolith acts as a cosmic beacon that traps living creatures, forcing them into an endless cycle of death and resurrection. The field refuses to let anyone leave alive because no human possesses the spatial coordinates needed to navigate its constantly shifting geometry. Travis realizes that anyone who touches the stone gains complete omniscience over the field, but at the cost of being permanently bound to its will. Ross touched the stone out of selfish arrogance and went insane, becoming its murderous herald. Travis, however, touches the stone out of pure love for Becky and their child. Because Travis touches the stone with a selfless intention, he retains enough sanity to open a doorway through time and space. Travis sends young Tobin back into the church moments before Becky and Cal ever set foot in the field. When Becky sees Travis's pendant around Tobin's neck, the physical proof stops her from entering the grass. By driving away with Tobin, Becky and Cal survive, saving Becky's baby and breaking the horrific cycle once and for all.