In 1992, a young girl named Rebecca Summers unexpectedly ventures into her family's pool during the night to retrieve a toy boat for her terminally ill brother, Tommy. However, a mysterious force pulls her beneath the water, resulting in tragedy. Fast forward to the present day, the Waller family—comprising Ray (Wyatt Russell), Eve (Kerry Condon), and their children Izzy and Elliot—is in search of a new permanent home following Ray's forced retirement from baseball due to health issues.
They purchase a house featuring a swimming pool, convinced that it will aid in Ray's recovery. While working on the pool, Ray accidentally scratches his hand, and upon inspection by the pool maintenance crew, they discover that it is self-sustaining, drawing from an underground spring. As Ray uses the pool for his therapy, his condition appears to improve. Nevertheless, Eve becomes increasingly worried about the unsettling transformation she observes in her husband.
As eerie occurrences start to mount, both Izzy and Elliot face dangerous encounters in the water, and their family cat mysteriously disappears. During a pool party, their realtor shares an unsettling story about Rebecca Summers drowning in the same pool. Tension escalates when Ray nearly drowns a child during a fit, initially attributed to his illness. Desperate to understand, Eve reaches out to the Summers family, discovering a dark history of disappearances linked to their new home.
Upon meeting Lucy (Jodi Long), Rebecca's mother, the horror unfolds. Lucy reveals that the pool was once part of a healing spring, with the grim twist that someone must be sacrificed for its powers to be utilized. Realizing that Ray’s recovery comes at a dire cost—the potential sacrifice of one of their children—Eve is filled with dread.
As events spiral further out of control, Ray becomes increasingly manipulated by the water's supernatural entity, entrapping Elliot and threatening Izzy. Courageously, Eve rushes to save her son while Izzy confronts the force controlling their father, ultimately striking him with a baseball bat. With great effort, Eve retrieves an unconscious Elliot from the depths, guided by Rebecca's spirit back to safety.
Amid the chaos, Ray regains his strength and clarity just in time to protect his children. In a selfless act of love, he sacrifices himself to the entity, ensuring their safety. Determined to keep the malevolent force from claiming any more victims, Eve, Izzy, and Elliot decide to fill in the pool, seeking to prevent the sinister history from repeating itself.
