The movie unfolds with the story of the Eve family, who move into a house that once belonged to the Summers family, known for a dark secret linked to their swimming pool. The narrative begins with a haunting incident from 1992, where a young girl named Rebecca Summers witnesses her toy boat in the pool and promises to retrieve it for her sick brother, Tommy. During her attempt, a mysterious force pulls her under the water, and only her bunny slipper surfaces, leaving her disappearance shrouded in mystery and establishing a sinister connection to the house.
In the present day, Ray, Wyatt Russell, the father who was a professional baseball player, struggles with a degenerative disease that has abruptly ended his career. The family, which includes Eve, and their children Izzy and Elliot, is searching for a new home. They visit the old Summers property, where Ray’s fascination with water therapy leads him to try to retrieve something from the pool, unknowingly putting himself in danger when he gets pulled into the water. Despite his struggle, he is rescued, and he later confesses to Eve that he believes practicing water immersion could help alleviate his illness.
As the family moves into their new home, the children quickly adapt to their surroundings. Izzy, played by [Kerry Condon], develops a crush on a boy named Ronin, and the kids settle into school routines. They decide to clean the pool, but Ray suffers a deep cut when he sticks his hand into the drain, raising concerns about the water source, which is surprisingly traced back to a nearby spring. Despite this, they begin using the pool regularly, with Ray claiming to feel better thanks to his therapy. However, strange occurrences soon follow. Eve uses the pool alone at night and believes she sees Ray standing above her underwater, but when she resurfaces, he's nowhere to be found. Their pet cat, Cider, mysteriously goes missing after cats tend to react anxiously around the pool, indicating something sinister lurking beneath the surface.
Elliot, their young son, also experiences unsettling visions. One day, he sees a collection of coins being thrown into the water and believes Izzy is bothering him. However, he soon hears Rebecca’s voice calling for his mother, accompanied by the appearance of her ghostly figure. Rebecca’s apparition offers Elliot a toy but then reaches out with a hand that pulls him underwater, forcing him to escape in terror. Elliot’s warning prompts concern, but Ray dismisses his fears, suggesting he might just be imagining things. This dismissiveness intensifies as strange events escalate.
During a little league practice, Ray begins to show signs of his condition, suddenly falling and having difficulty moving. Nevertheless, he surprises everyone by performing brilliantly at the game, smashing a ball so powerfully it knocks a light down, much to the villagers’ cheers. However, Elliot notices something worrying about Ray's behavior, hinting at deeper issues. Izzy invites Ronin over for a pool day, where what begins as fun quickly turns frightening. During a game of Marco Polo, Izzy loses sight of Ronin, who later growls and pulls her underwater in a grotesque, bloated creature’s grasp—an entity tied to the pool’s dark past. Izzy manages to break free, but her experience leaves her feeling unwell, and she admits to Elliot that something terrible lurks in the water.
A neighborhood pool party is held, bringing many residents together. Meanwhile, Eve uncovers disturbing truths about the property's history. The realtor admits the house was never properly disclosed and reveals that other residents, including Rebecca and Tommy, had vanished without explanation. Eve’s investigation leads her to Rebecca's mother, Kay, who is now suffering from an illness similar to her son's. Kay confesses that the water from the pool originates from an ancient spring tied to a well that grants wishes—at a terrible cost. She reveals that Rebecca was sacrificed to satisfy an evil entity in the water, which now possesses her, causing her to regurgitate black, tar-like water.
Realizing her children are in grave danger, Eve frantically tries to protect them. That night, alone in the house, Elliot hears strange noises and goes outside, where he falls into the pool after a scare involving the ghostly pool demons. The pool cover begins to seal itself, trapping him. Inside, a possessed Ray attacks Izzy. Meanwhile, Eve plunges into the water to rescue Elliot. She fights against ghosts and spirits, including Rebecca’s previous victims, who try to drag them under. Eventually, Izzy, with bravery, manages to fight off her possessed father, forcing him to vomit the black water that clung inside him, and Elliot regains consciousness.
As the entity grows more aggressive, Ray makes the ultimate sacrifice, walking into the water to save his family from the malevolent force. His selfless act frees Elliot from the entity's grip, but at great cost. The family mourns Ray’s sacrifice, recognizing his bravery in fighting the evil that had haunted the house for decades. They decide to leave the house permanently, sealing and burying the pool with dirt to destroy the creature once and for all, ending its cycle of death and despair. Through their ordeal, they learn the importance of facing their fears and standing together against dark forces that threaten innocence.
