Bonnie Emily Hudson is the daughter of Lord and Lady Willoughby, who live on the dignified country estate Willoughby Chase. With Lady Willoughby ill, the couple plan a restorative voyage to the Mediterranean, hoping the sun and sea will steady her health. In London, Bonnie’s cousin Sylvia Aleks Darowska prepares to keep Bonnie company while her parents are away, a duty that also ties her to their impoverished Aunt Jane. On the train to the Willoughbys’ home, Bonnie and Sylvia cross paths with a mysterious man, Mel Smith in the role of Mr. Grimshaw, whose presence feels at once curious and unsettling.
At Willoughby Chase, a striking new governess arrives to supervise the girls: Letitia Slighcarp [Stephanie Beacham], a poised and stern middle‑aged woman whose cordial exterior masks a calculating mind. The next morning, Bonnie rides out in a carriage to fetch Sylvia, and upon a journey back to the estate, Grimshaw is knocked unconscious when a wolf attack disrupts the train. The sisters manage to bring him back to the house, while the Thessaly, the Willoughbys’ ship, sails toward its fate. The following day, Bonnie and Sylvia are out in a sleigh when wolves threaten them again, only to be rescued by Simon, a boy who lives in a nearby cave. They return to find the staff abruptly reduced to a lean handful: James [Richard O'Brien] and Pattern [Jane Horrocks] are all that remain among the servants, while Slighcarp refuses to explain the strange purge.
Bonnie tries to reason with Slighcarp about the changes, but the governess’s behavior grows increasingly harsh. The girls discover Slighcarp wearing Lady Willoughby’s best dress, and Bonnie confronts her, splashing water in anger. In retaliation, Slighcarp imprisons Bonnie in a cupboard, and Sylvia sees James being ordered to remove all the toys from the house. Sylvia steals the key from Slighcarp, who is bathing, and frees Bonnie. From a hidden corner, the girls overhear Slighcarp reveal a plot: she has forged a copy of Lord Willoughby’s will with Grimshaw’s help to claim the fortune, and she has arranged the sinking of the Thessaly to cover her tracks. Grimshaw hurls the true will into the fireplace, but Bonnie and Sylvia salvage part of it, and Sylvia locks Bonnie back in the cupboard to avoid discovery. In a bid to buy time, Bonnie feigns madness to lure in the local physician, Dr. Morne [Jiří Lábus], but the plan falters when Slighcarp burns a letter addressed to him.
Slighcarp condemns the girls to life at Blastburn, an industrial town’s orphanage run by Gertrude Brisket [Geraldine James] and her teenaged son, Rupert [Abbie Dabner]. The two girls must work in a dangerous laundry attached to the facility, where machinery is finicky and deadly. Sylvia narrowly escapes disaster when Rupert orders her to repair a high catwalk above a crushing machine, and a small boy named Joey [William Martin], who has tried to help, dies after slipping into a laundry tub. Bonnie publicly challenges Brisket about the brutal conditions, a defiant stand that earns them harsh punishment: confinement in the coal cellar. Slighcarp learns that the Thessaly has sunk and returns to Blastburn, while Simon secretly slips into her carriage, evading capture.
A daring escape attempt follows as Simon helps Bonnie and Sylvia, but they are discovered. Rupert and Slighcarp pursue them through the laundry, and Rupert falls to a fatal end when crushed by a mangled mechanism on the catwalk. The trio manages to flee in Slighcarp’s carriage, making it back to Willoughby Chase, with Brisket and Rupert left behind. Slighcarp and Brisket pursue in a motorized sleigh, but Brisket is pushed from the vehicle and quickly overwhelmed by wolves. Slighcarp’s sleigh later loses control and explodes, leaving her fate in doubt—yet she is never far behind as danger rages on.
Back at the manor, justice closes in: James learns the Willoughbys are alive, and with Pattern’s help, they capture Grimshaw as he attempts to flee. Bonnie and Sylvia return to the house, only to find wolves roaming the grounds, forcing them toward Simon’s cave as a last refuge. Slighcarp intercepts them at the cave’s entrance, and a final confrontation unfolds. Although Slighcarp survives the explosion earlier, her face bears burns and her clothes are singed; she menaces the girls, attempting to destroy the remaining portion of the original will. In a desperate act, Bonnie pushes Slighcarp into the stove and sets her dress aflame, sending the villain fleeing into the snow with a scream.
Meanwhile, Bonnie’s parents reveal they survived the sinking of the Thessaly and were stranded on a tropical island, from which they eventually return home. Grimshaw is arrested for attempting to steal the estate’s silver, and Lord Willoughby decides that Aunt Jane should have a secure place to live: the mansion’s East Wing, ensuring that Sylvia does not have to leave her cousin behind. With the threat of Slighcarp finally neutralized and the family restored, the Willoughbys begin to rebuild their lives, trusting in their bonds of family and the quiet strength of those who helped them endure the perilous months.
