Becky, who is six months pregnant, and her brother Cal DeMuth embark on a journey to San Diego. Their plan is for Becky to stay with her aunt until she gives birth so she can put her baby up for adoption. However, their trip takes a dark turn when they stop by an old church and hear a young boy named Tobin calling for help from a field of tall grass. As they investigate, they also hear his mother, Natalie, pleading with him not to seek any assistance.
Worrying for the boy's safety, Becky and Cal enter the grass, but soon they find themselves separated as the terrain behaves in bizarre and unsettling ways. Panic escalates, and their attempts to leave become futile as darkness descends. Amid the confusion, Cal stumbles upon a frightened and dirty Tobin, who oddly holds a dead crow. Tobin shares that the grass does not allow dead things to move, burying the bird in the ground as they converse. During a tense moment, Becky encounters Tobin’s father, Ross, but they too become lost in the endless grass.
As events unfold, Cal is led by Tobin to an ancient stone, but their moment is interrupted by Becky’s chilling scream as she is attacked by an unseen menace. Meanwhile, Becky's child’s father, Travis, arrives, looking desperately for both Becky and Cal. He discovers their abandoned vehicle and enters the field, ultimately meeting Tobin, who cryptically leads him to Becky’s lifeless body. Stricken by grief, Travis is further haunted by Tobin's presence, who seems to exist in different timelines as they navigate the menacing field.
The following day, confusion reigns as a much earlier version of Tobin arrives at the church where his family gathers. The family's dog, Freddy, runs into the grass, prompting panic and a frantic search as Travis calls for Tobin. Unfortunately, this leads to further disarray as Ross, driven by a growing sense of dread, discovers the ancient rock. Upon touching it, he becomes detached from reality, deepening the peril for everyone involved.
The narrative thickens as Becky receives a distressing phone call warning them against repeating mistakes. As their journey proceeds, Becky experiences a horrific event where the grass seems to enter her womb, rendering her unconscious. Fortunately, Ross revives her moments later, suggesting a way out of the ominous field. However, revelations about the rock arise when Natalie warns them of its danger, igniting conflict among them. Desperate to escape, they suffer losses, including the deaths of Natalie and injuries inflicted on Travis.
In a tragic twist, the group manages to find refuge in an old building where they discover that Freddy, the dog, has escaped through a hidden passage leading to safety. Competing feelings of jealousy and desperation surface when Cal allows Travis to slip to his doom during a tumultuous encounter. Tobin, witnessing these events, flees for safety while Cal struggles with his fate, revealing a perilous time loop in which he is pursued relentlessly by the unhinged Ross.
As the plot develops, Becky engages in a cat-and-mouse game with Ross, managing to inflict an injury before encountering monstrous beings that threaten her. In her attempts to reach out to her former self, she becomes ensnared in a web of visions and hallucinations that blur reality. Tragically, she soon learns that her brother was not who he seemed, revealing a shocking connection to Ross.
In a heart-wrenching finale, while collapsing under the agony of betrayal and violence, the critical realization dawns that only through understanding the ‘grass’ can the characters break the cycle of despair. Ultimately, after being confronted by Ross, with everything on the line, Travis takes a gamble, touches the stone, and confronts the visions that have haunted them. With determination, he manages to guide Tobin out of the grass, urging him to prevent Becky and Cal from entering again.
Finally emerging in the church, armed with the knowledge that will seal their fates, Tobin prevents them from stepping into the grass—closing the loop and allowing Becky to decide to keep her baby as they drive away. Travis, listening from the field, finds peace as he succumbs to the grass. Through this haunting odyssey, the themes of fate, sacrifice, and the harrowing journey of acceptance intertwine, leaving a chilling impression of the darkness woven into the fabric of their lives.
