The king of the castle (or, in this case, rambling country manor), Sir William, is murdered in the middle of the night - twice. First poisoned, then stabbed, Sir William's death is an immediate scandal among his well-mannered guests, all of whom are kept at the estate over the weekend in a sort of unofficial house arrest while police trespass on to their carefully manicured world and begin making enquiries.
While Sir William's death sparks the principal action of the film, it's the intricate study of the world the characters inhabit that really makes the film worth watching. The focus switches equally back and forth between the "upstairs" and "downstairs" of the house in question. The lives of the servants are examined in as much (if not more) detail than those of the landed gentry they serve. The result is an honest assessment of the ways in which the two classes both feed on and rely on the other.
While the film is superb in its entirety, it truly excells in the first act. Here, it's "business-as-usual" at Gosford Park, and the film provides an amazingly detailed look at the day-to-day activities of the time. The camera captures the dizzying whirl of activity downstairs, as servants prepare meals and clean up after them, keep clothes and jewels at the ready, clean house, and generally enable their employers to live lives of unimaginable boredom. Throughout the barely-controlled chaos, tension rises until, by the time the murder finally occurs, it's so sharp you could cut it with an expertly polished butter knife.
The principal character (if there is one in this large, ensemble cast), is Mary Maceachran, a young, Irish lady's maid on her first job. She's employed by Constance, Countess of Tentrahm. Constance is a gossip-mongering snob who won't associate with anyone who doesn't employ a lady's maid and continually hounds Mary for juicy secrets about her fellow upper-crusts.
While doing her best to perform her endless stream of duties for her new mistress, Mary also develops an attraction to fellow servant, Robert Parks. The sexual tension between them (barely noticeable at first) rises steadily throughout the film. The characters continually dance around each other, neither one confronting the other until one of the final scenes of the film.
Mary also acts as a sort of unofficial investigator into Sir William's untimely demise. Her constant questions are viewed by the other characters only as the annoying pestering of an incompetent new maid. But throughout the film, Mary carefully narrows down the possible suspects and eventually figures out "whodunit."
And figuring that out is no simple task. Everyone, it turns out, has ample motive to commit the murder, from the wealthiest of Sir William's guests down to the lowest footman. In this dense, atmospheric world, secrets are layered upon secrets and no one is who they claim to be.
A major theme of the film is the gap of understanding created by the British class system. The lives of servants and employer are so intertwined that any action, committed by either servant or employer, sets off an entire cascade of events effecting the lives of everyone within the house. Yet the characters themselves remain painfully clueless, blinded by their own sense of self-importance. When veteran housekeeper Mrs. Wilson asks the police inspector (of no particular rank or privelege) when he wants to question the servants about Sir William's death, he responds in a condescending tone that he only wants to speak to people with "a real connection to the dead man."
The film touches on the lives of the very rich, but is primarily about the support system that makes such a life possible. This focus makes the film unique, as the place of servants in fiction largely reflects their position in real life. In most period pieces, servants exist perpetually in the shadows. In Gosford Park, servants take center stage and the story is stronger for it.