The narrative revolves around Ellie, a rebellious teenager who often finds herself at odds with her mother, ultimately choosing to leave home to stay with a friend. Meanwhile, we are introduced to Paris, a young man haunted by the traumatic memory of a car accident that claimed his mother's life when she drove off a bridge, although he miraculously escaped that fate.
Ellie resides near the enigmatic Caddo Lake, an eerie body of water lying on the border of Louisiana and Texas, where she lives with her mother, stepfather, and her eight-year-old stepsister, Anna. One fateful night, Anna mysteriously disappears while near the lake, setting off a frantic search by her family and the community at large. In a desperate attempt to find her sister, Ellie ventures to an area of the lake where the water has receded, revealing the dry lake bed beneath. Upon returning to her home, Ellie experiences sheer panic as she discovers that she has inexplicably traveled back three days in time.
Driven by urgency, she races back to the lake and astonishingly finds herself a month in the past—where Anna had previously run away. It soon becomes clear that this unusual phenomenon of time displacement is triggered by the fluctuation of Caddo Lake’s water levels. Undeterred by her predicament, Ellie continues her quest to find Anna, fueled by determination.
On a parallel storyline, we have Paris, who is on a mission to understand the circumstances surrounding his mother's seizure that led to the fateful accident. He works to dredge the lake, where he starts to notice bizarre occurrences, such as mysterious footprints along the dry lake bed. Compounding his confusion, he suddenly loses his hearing, and upon examining his boat, he uncovers a necklace—one that belonged to his mother. Just as a spark reignites between him and his former girlfriend, her skepticism regarding the lake's peculiarities creates a rift between them.
The two narratives intertwine when Paris travels back to 1952, where he encounters a wounded Anna and assists her, all while trying to navigate his way back to the year 2003. In a surprising twist, he lands in the present year of 2022, where Ellie is actively searching for her sister. During her investigation, Ellie finds a striking necklace in a grocery store parking lot that resembles her mother's pendant. When she learns that this necklace belonged to Anna, her emotions spiral. In a shocking moment, she discovers that the woman claiming the necklace is her mother, Celeste, holding a baby who is revealed to be Ellie herself. Stunned and bewildered, Ellie races away to find an internet café.
Once there, she researches Anna Lang, the name associated with the necklace, and stumbles upon old photographs that alter her understanding of her family; it dawns on her that her beloved stepsister, Anna, is actually her grandmother. With newfound resolve, Ellie hurries back to the lake, desperate to return home.
Amidst Ellie's turmoil, Paris is apprehended by authorities while waiting in Ellie's boat, which she had secured before her temporal journey. He soon learns from a news report that the girl he saved in the past was indeed his mother, which sends him into a panicked frenzy. Although he attempts to escape from the police, tragedy strikes as he drowns while trying to traverse back through the portal to his original timeline.
Returning to 2022, Ellie grasps the shocking truth that her stepfather, with whom she has been at odds, is ironically her great-grandfather. She rushes home only to find her mother in a state of shock, glued to the television screen watching a report about Paris's eerie escape and untimely death. The broadcast features his photograph and a plea for information from the public. In an emotional climax, Ellie presents her mother with Anna's old school picture from 1952, wrapping her arms around her as she reveals the heart-wrenching reality that neither Anna nor Paris ever intended to leave them.
