In 1969, four British siblings—Jack, Jane, Billy, and Sam Marrowbone—flee to America with their mother, Rose, to escape their troubled past. The mother and her children take refuge in her abandoned family home, a remote mansion in rural Maine. This new world promises a fresh start, away from the darkness that had engulfed their previous lives. Unfortunately, their hopes for a peaceful life are shattered when Rose falls ill and passes away, leaving the siblings alone and facing a bleak future. To prevent the authorities from splitting them up, the eldest, Jack, insists they keep their mother's death a secret until he comes of age to legally care for them. The siblings agree to hide their situation and maintain a facade of normalcy to the outside world.

As they forge a simple life together, the children befriend a local girl, Allie, who becomes a beacon of hope and love, particularly for Jack. However, their fragile stability is soon threatened by both external and internal forces. The children find themselves haunted by a sinister presence that may be linked to the dark history of the Marrowbone house. This ominous specter casts a shadow over their existence, turning their safe haven into a place of paranoia and dread. The siblings' loyalty and resilience are put to the test as they grapple with the escalating terror within their walls, while attempting to keep the outside world at bay.