The tranquility shatters when Lord Harwood, the stern and secretive patriarch, is found dead in his study on Christmas Eve, his pocket watch smashed and his desk in disarray. The initial assumption is that his failing health finally claimed him, but Miss Marple quickly notices inconsistencies: the angle of the fallen chair, the overturned glass that had not spilled, and a faint scent of cigar smoke belonging to someone who was not the victim. With the snowstorm isolating the Hall from the outside world, it falls to Marple to unravel the truth before fear and suspicion consume the household.

As she begins to investigate, Miss Marple observes the fractured dynamics among the family and guests. The eldest son, Richard, harbors a desperate need for approval that curdles into bitterness. The widowed daughter-in-law, Eleanor, is quietly terrified of something she refuses to name. A charming yet elusive cousin, Felix, seems to know more than he lets on. Even the loyal house staff display unusual nervousness, suggesting that loyalties may not be as steadfast as they appear. Marple, with her gentle demeanour and razor-sharp intuition, listens, watches, and connects threads the others overlook.

Complications mount when a second incident occurs—a mysterious fire set in the old servant’s wing, clearly intended to destroy evidence. The remaining guests panic as the snow deepens and outside help remains impossible. Marple pieces together clues hidden in old letters, overheard quarrels, and inconsistencies in alibis. She uncovers long-buried grievances tied to a decades-old scandal involving a forbidden romance, a missing heirloom, and a betrayal that fractured the Harwood family. Each revelation tightens the web of suspicion, yet the truth remains just out of reach.

In a dramatic Christmas morning confrontation, Miss Marple gathers everyone in the drawing room, revealing that Lord Harwood’s death was neither sudden nor natural. She explains that the smashed watch was meant to falsify the time of death, that the fire was a last-ditch effort to erase a damning document, and that the murder was driven not by greed but by a heartbreaking attempt to prevent the exposure of a family secret. Her calm, precise deductions lead to the unmasking of the culprit, whose motives are rooted in love twisted by fear. The confession shocks the household, yet it also brings long-awaited truth into the open.

The film concludes with the storm clearing and peace slowly returning to Winterbourne Hall. Though the family remains shaken, Christmas ends on a muted yet hopeful note as they begin to reconcile with the past and with one another. Miss Marple, ever composed, prepares to leave with quiet satisfaction, having restored order in the midst of chaos. As she steps out into the crisp winter air, the chimes of the repaired Hall clock ring out, symbolizing both closure and the gentle persistence of justice.