A man, desperate and driven by tragic circumstances, takes the lives of his business partners and estranged ex-wife before fleeing with his daughters, the young Victoria and infant Lilly. Their harrowing escape leads them down a secluded, snowy road, where fate intervenes as he crashes the car into the woods. Seeking refuge, he discovers an abandoned cabin, which soon becomes the setting of a dark turn of events. As he grapples with the gravity of his actions and plans an unimaginable end to their suffering, a mysterious figure interjects at the last moment, resulting in his untimely demise. Victoria, still reeling from the accident and wearing broken glasses, can only perceive this stranger as a blur.
Fast forward five years, and their Uncle Lucas, portrayed by Nikolaj Coster-Waldau, along with his girlfriend Annabel, has been tirelessly searching for the missing siblings, utilizing their inheritance as a means to fuel their quest. Their efforts lead them to the abandoned vehicle and ultimately to the cabin, where they find Victoria and Lilly, who have reverted to wild, animalistic behavior, with Victoria instinctively protecting Lilly from these newfound intruders.
Cared for by the enigmatic psychiatrist Dr. Dreyfuss, the children enter a struggle over custody, with Lucas facing an uphill battle against a wealthy relative who wishes to claim the girls. The court questions their capability to provide a stable upbringing, particularly given their unique needs. However, Dr. Dreyfuss agrees to back Lucas and Annabel's guardianship provided they relocate to a specialized facility. Unknown to them, the ominous presence of the entity the girls refer to as "Mama" has followed them into their new home.
Gradually, the history of "Mama" unfolds through a mix of haunting dreams experienced by Annabel, Dr. Dreyfuss's research, and the children's accounts. The figure is revealed to be a tragic soul, once a patient in an 1800s asylum. Her heartbreaking past is marked by the loss of her own baby, Lily, after a series of harrowing events led her to leap off a cliff in despair. Now, as an apparition, she seeks solace and attempts to connect with the abandoned children who remind her of her lost offspring.
Her presence, often foreshadowed by ominous black butterflies and electrical disturbances, leads to numerous frightening encounters. Tension rises as Lucas, now pushed into a coma due to Mama's interference, leaves Annabel reluctantly stepping into a protective role for both girls. As time progresses, Victoria begins to acclimate to the family unit, while Lilly remains fiercely loyal to Mama, raising suspicions in Annabel as she grapples with the reality of the supernatural threat.
Dr. Dreyfuss eventually discovers unsettling truths regarding Mama and, in his quest for understanding, meets a gruesome fate at her hands. Meanwhile, the children's aunt, concerned for their welfare, begins to monitor the household more closely, convinced Annabel has abused the girls. Lucas, guided by visions from the afterlife, seeks to rescue his nieces as Annabel discovers the dark truths hidden in Dr. Dreyfuss's belongings. The tension escalates as Mama's rage culminates in a terrifying confrontation leading Annabel and Lucas to the cliff, mirroring Mama’s tragic past.
In a heart-stopping finale, as they rush to save the children from a fate resembling their own twisted history, a pivotal moment occurs when Mama finds her baby’s remains, causing her form to momentarily revert to something more human. Ultimately, the scene becomes a tragic echo of the past as Mama believes that reunion with her child lies in death. In a desperate plea for survival, Victoria calls out to Lilly, but as Mama and Lilly leap together into the abyss, they transform into a beautiful storm of butterflies, leaving Victoria with a bittersweet realization that one of them might very well be Lilly, hauntingly connecting her to the past.
