In November 1932, a prosperous English industrialist, Sir William McCordle, along with his wife, Lady Sylvia, and their daughter Isobel, hosts an extravagant weekend shooting party at their lavish country estate, Gosford Park. A mix of guests arrive, including Sylvia's sisters, Louisa and Lavinia, along with their spouses, Lord (Raymond) Stockbridge and Commander Anthony Meredith; their aunt Constance, the Countess of Trentham; the Honorable Freddie and Mabel Nesbitt; the renowned actor Ivor Novello; and American film producer Morris Weissman. The party also sees the late arrivals of Lord Rupert Standish and Jeremy Blond.
The housekeeper, Mrs. Wilson, organizes the visiting staff and takes special note of Robert Parks, the valet of Lord Stockbridge, who reveals his background in an orphanage. The inexperienced head maid, Elsie, mentors newcomer Mary MacEachran, who serves Lady Trentham in navigating the evening’s events.
After a sumptuous dinner, the gathering is thrown into disarray when a silver carving knife goes missing. Henry Denton, Weissman's valet, creates tension among the staff with his probing inquiries and sneaks off for a late-night rendezvous with Lady Sylvia. Meanwhile, Isobel implores Elsie to persuade Sir William to hire Freddie, who is using Isobel's past affair and her aborted pregnancy to blackmail her. Freddie, who married Mabel for her presumed wealth, mistreats her, while Rupert tries to win Isobel's affections.
Lady Trentham confides in Mary about a long-standing rivalry between Sylvia and Louisa, who once cut cards to determine who would marry Sir William. As the men go out for pheasant shooting the following morning, a stray shot narrowly misses Sir William. During lunch, the ladies join them, but Sir William retracts his support for Anthony's business venture, leaving him financially devastated.
Later, Lady Sylvia warns her aunt that Sir William might stop Constance's financial allowance. During dinner that evening, tensions rise when Lady Sylvia chastises Sir William, prompting Elsie to defend him, which inadvertently reveals their affair. Elsie leaves in shame while Sir William, angry, storms into the library after knocking over a coffee brought by Mrs. Wilson. As the guests gather in the drawing room to listen to Novello play piano, several of the men discreetly slip away. One of them, only identifiable by his trousers, secretly retrieves the missing knife and enters the library, where he brutally stabs Sir William, who is slumped over in his chair.
The shocking discovery of the body triggers the arrival of the clumsy Inspector Thompson and the sharp-witted Constable Dexter to investigate the murder. Meanwhile, Henry finds his way back to Lady Sylvia for more clandestine encounters, revealing he is actually an American actor cleverly masquerading as Weissman's valet for role preparation.
The plot thickens when it is revealed that Sir William had been poisoned prior to his stabbing. Mrs. Croft, the head cook, shares sordid stories about Sir William and his mistreatment of female factory workers. Isobel attempts to buy Freddie's silence with a cheque, which only escalates tensions with Mabel. Inspector Thompson hastily releases guests without thoroughly questioning the staff.
In a revealing confrontation, Mary discovers Robert's motives; he had become Lord Stockbridge's valet to get close enough to Sir William for revenge. Robert discloses that he is actually Sir William's illegitimate son, abandoned at an orphanage after his mother's death. He denies poisoning Sir William, while Mary points out that the cause of death was indeed poison. They share an intimate moment with a kiss, yet their complicated families loom large over their connection.
As the event winds down and the guests depart, Freddie seeks to team up with Anthony, while Isobel turns down Rupert's advances after overhearing him plot about her inheritance. Elsie climbs into a carriage headed for London with Weissman, Novello, and Henry.
In a final conversation, Lady Trentham and Lady Sylvia discuss the long-standing feud between Mrs. Croft and Mrs. Wilson, leading Mary to an important realization: Mrs. Wilson is Robert's mother. When confronted, Mrs. Wilson reveals that she and Mrs. Croft bore Sir William's children while working in his factory, one of whom she lost, and the other she gave up. Determined to protect Robert, she poisons Sir William, ensuring that he would only be guilty of stabbing a corpse. As the film concludes, Mrs. Croft offers comfort to Mrs. Wilson, while Mary bids farewell to Robert, leaving their shared past unspoken as the party guests go their separate ways.
