Constance, Countess of Trentham is a snobbish and toffee-nosed aunt of Lady Sylvia McCordle. Her late husband, the Earl of Trentham has not left her any money after his death and because of that Sir William McCordle, Lady Sylvia's husband, has to pay an allowance to the "old mother" Trentham.
When McCordles arrenge a shooting party in November 1932 at their country mansion the guest list includes, of course, Aunt Constance, who, by the way, hates shooting. Like all other guests, Constance has taken her maid to take care of her and Constance's maid Mary is the main character of the film.
Constance, whose all pleasures come from food, gossips and Schadenfreude, starts her visit to Gosford Park by complaing about her small allowance. Sir William, who everybody dislikes, gets angry and says he will stop paying alimony to Constance at once. Constance panics, but it is in vain, because in following night, Sir William is murdered. Constance has an alibi (she's been playing bridge with other guests), but still the stupid inspector Thompson thinks that Constance is a possible murderess. Mary the maid lies for her mistress, but after all it is clear, that Constance is not guilty, but the housekeeper of Gosford Park is.
After William's death Constance's future is saved because now her mainentance will be paid from money that William left behind. With everybody else, Constance leaves Gosford Park with happy face, though she's worried if she has to testify in the court.