In 1673 Mexico, a beautiful woman drowns her two sons in a river in a fit of jealous rage after discovering her husband's infidelity, immediately drowning herself in regret and becoming the cursed spirit La Llorona.

Three centuries later, in 1973 Los Angeles, widowed child services caseworker Anna Tate-Garcia investigates the home of Patricia Alvarez. She finds Patricia's young sons locked behind a heavy door etched with protective sigils. Believing Patricia is abusing her children, Anna frees the boys and places Patricia under police custody despite the mother's frantic warnings that locking them away was the only way to shield them from a weeping woman. Later that night, the two boys wander toward a nearby storm drain and are drowned in the river by La Llorona.

Anna is called to the crime scene, bringing her young son Chris and daughter Samantha along in the car. While waiting, Chris hears weeping and steps outside, encountering the spirit of La Llorona, who grabs his wrist and burns a demonic mark onto his skin. Soon after, Samantha is also attacked in the backyard by the pool, receiving an identical burn. Patricia, consumed by grief, accuses Anna of causing her sons' deaths and reveals she prayed to La Llorona to take Anna’s children instead.

Recognizing that the bruises and supernatural occurrences in her home are not hallucinations, Anna seeks the counsel of Father Perez. Perez recounts his own terrifying past encounter with a possessed conduit doll (Annabelle), explaining that the formal church bureaucracy is too slow to help them in time. He directs Anna to Rafael Olvera, a former priest who operates outside church jurisdiction as a folk shaman. Rafael agrees to fortify Anna's home, using dried seeds from a fire tree that grew by the river where La Llorona drowned her sons.

Rafael sets a protective perimeter inside Anna's house, spreading crushed fire-tree seeds across door sills and windows to prevent the weeping woman from crossing. The night turns into a harrowing siege as La Llorona exploits household vulnerabilities, manipulating wind, shattering glass, and luring Samantha to break the seed barrier to retrieve her doll.

Patricia Alvarez storms the house armed with a gun, seeking to hand Anna’s children over to the spirit in exchange for her own deceased boys. However, seeing the genuine terror of Chris and Samantha reminds Patricia of her maternal instincts, and she chooses to help Anna and Rafael fight back.

In the climactic confrontation in the attic, Rafael blesses pool water with his rosary, throwing it at the apparition to weaken her. Anna arms herself with a sharpened wooden cross crafted from the branches of the original fire tree. As La Llorona lunges forward with a deafening screech, Anna drives the wooden cross straight through the spirit’s chest. The cross shatters La Llorona’s physical manifestation into dissolved ash and smoke, breaking the curse and saving the children. Rafael bids the family farewell, leaving them safe in the morning light.