Just a few hours before U.S. Presidential hopeful Senator Robert F. Kennedy's 1968 assassination, party-goers, performers, hotel employees, and campaigners all descend on the hotel in preparation for the big night. They include the Ambassador's retired doorman (Anthony Hopkins) who plays chess in the grand lobby with fellow retiree Nelson (Harry Belafonte); the hotel's current manager, Paul Ebbers (William H. Macy) whose wife Miriam (Sharon Stone) is the hotel's hairdresser; the stifled hotel switchboard operator Angela (Heather Graham) who hopes her affair with Ebbers will lead to a promotion, to the dismay of her co-worker Patricia (Joy Bryant).

The hotel's kitchen workers include the bigoted boss Timmons (Christian Slater), chef Edward Robinson (Laurence Fishburne); Latino workers Jose (Freddy Rodriguez), who would rather be watching the night's pivotal Dodgers baseball game, and Miguel (Jacob Vargas), and coffee shop waitress Susan (Mary Elizabeth Winstead), newly arrived from Ohio, hoping to become a big star.

Meanwhile, the hotel's guests include alcoholic singer Virginia Fallon (Demi Moore), who is scheduled to introduce Senator Kennedy at his California Primary party, and her frustrated husband Tim (Emilio Estevez); a young bride-to-be (Lindsay Lohan), who's about to marry a young man (Elijah Wood) in order to save him from going to Vietnam; and a depressed socialite from the East Coast (Martin Sheen) and his much younger wife (Helen Hunt) who are in California on a strained second honeymoon.

Also gathered in the Ambassador are Kennedy campaign followers, including devoted young aides Wade and Dwayne (Joshua Jackson, Nick Cannon); persistent Czech journalist Lenka (Svetlana Metkina); and novice volunteers Jimmy and Cooper (Brian Geraghty, Shia Lebeouf), whose day of campaigning is radically changed when they run into a drug dealer (Ashton Kutcher) who initiates them into the infamous acid trip experience.

As the day progresses, each of these characters will encounter their own battles between sexes, races, and social classes, between personal despair and public hope as they all converge on the ballroom for Kennedy's speech, never to be the same again.