It's November 1932 and various family members and friends have descended upon Gosford Park - a sprawling country estate owned by Sir William McCordle (MICHAEL GAMBON) and his wife, Lady Sylvia (KRISTIN SCOTT THOMAS) -- for a shooting party. Among them is their daughter, Isobel (CAMILLA RUTHERFORD); her haughty great aunt, Constance (MAGGIE SMITH); and Sylvia's sisters Louisa (GERALDINE SOMERVILLE) and Lavinia (NATASHA WIGHTMAN) and their husbands, Raymond (CHARLES DANCE) and Lt. Cmdr. Anthony Meredith (TOM HOLLANDER).
Then there's Freddie Nesbitt (JAMES WILBY) who married Mabel (CLAUDIE BLAKLEY) only to discover that her family money was limited; Lord Rupert Standish (LAURENCE FOX) who's courting Isobel, and his friend Jeremy Blond (TRENT FORD); as well as Ivor Novello (JEREMY NORTHAM) a famous actor, and Hollywood producer Morris Weissman (BOB BALABAN) who makes Charlie Chan pictures.
Meanwhile, the "below stairs" staff of butlers, maids, cooks and valets try to make sure everything goes smoothly for their employers. Among them is Jennings (ALAN BATES), the head butler who runs the manor with Mrs. Wilson (HELEN MIRREN), the housekeeper. Mrs. Croft (EILEEN ATKINS) runs the kitchen and doesn't get along with Mrs. Wilson, while Elsie (EMILY WATSON) is the head housemaid who's having an affair with Sir William.
She takes Mary Maceachran (KELLY MACDONALD), Constance's maid who's new to serving others, under her wing and shows her the ropes that includes ignoring the peccadilloes of other staffers such as Bertha (TERESA CHURCHER) who's occasionally caught in compromising positions. Meanwhile, visiting valets Robert Parks (CLIVE OWEN) and Henry Denton (RYAN PHILLIPPE) - who work for Raymond and Morris respectively - have aroused different reactions from the rest of the staff. While some of them, such as first footman George (RICHARD E. GRANT) become increasingly suspicious of Henry, Robert's guarded comments on growing up in an orphanage elicit some sympathy from the others.
None of that emotion is directed at Sir William, however, as most don't like or get along with him. Not surprisingly, tensions mount and someone ends up dead, resulting in the arrival of buffoonish Inspector Thompson (STEPHEN FRY) and his more efficient assistant, Constable Dexter (ROB WEBSTER) who investigate the murder.
As they attempt to discover exactly what led up to the act and the identity of who's responsible, the various members of the above and below-stairs worlds react to that and their various personal issues.