NOTE: This spoiler was sent in by Alison who explains that due to the jumping around so much, she couldn't write it in order but rather, by the individual storylines.
The whole movie is about how a group of people in the Ambassador Hotel converge on the place and the day RFK was shot there. The movie establishes the scene with a bunch of historical montages.
John Casey (Anthony Hopkins) who used to be the doorman there spends the entire day talking with and playing chess with friend, Nelson (Harry Belafonte) and tells him about all the famous people he has greeted at the Ambassador hotel. Nelson tells him that he feels he is getting too old.
A kitchen worker, Jose (Freddy Rodriguez), who is working a double shift and can't take his father to the game where Don Drysdale pitches his sixth shut out (a record) he discusses with his friend Miguel (Jacob Vargas) and another cook, Edward (Laurence Fishborne), tells him not to be so angry about racial discrimination, that the trick is to let the others think it was their idea. Jose gives his tickets to Edward and Edward calls him a king, writing "The Once and Future King" on the kitchen wall. Jose listens to the game on a radio.
The kitchen manager, Timmons (Christian Slater) starts off getting fired (with a couple weeks notice) by the hotel manager, Paul (William H. Macy) partially for not telling the workers they could have the day off to vote.
A hotel operator, Angela (Heather Graham) tells another operator, Patricia (Joy Bryant) about an affair she is having with a married man, who turns out to be Paul. As she is going into the hotel room, Timmons observes her and puts two and two together. Later the Angela ends it with Paul.
Meanwhile, the workers at the Kennedy campaign are busy canvassing and making calls, hoping to win the primary. The main guy, Wade (Joshua Jackson) tells another one, Dwayne (Nick Cannon), who we've been following all day, that Robert Kennedy has personally asked to meet with him. He also encounters two campaign workers (Shia LaBeouf Brian Geraghty), who don't want to ride on the bus so they blow off campaigning
The two campaign workers go to buy a joint from the hotel's local drug dealer (Ashton Kutcher), who instead convinces them to try LSD. The two first go through a long hallucination scene and then spend the rest of the day high doing strange things, as they have never done acid before.
Meanwhile, Virginia Fallon (Demi Moore) is performing nightly at the hotel, that night being her last performance. Virginia Fallon is drinking a lot and talking about future shows and it becomes pretty clear that her husband, Tim (Emilio Estevez) is upset with the whole situation. He tells her not to embarrass them and be too drunk to introduce Robert Kennedy that night. He then goes and plays with the band again, as he used to be a drummer.
Meanwhile a young woman, Diane (Lindsay Lohan) comes into the hotel beauty parlor to get her nails done for her wedding that day. The hairdresser Miriam (Sharon Stone), who is married to Paul, talks to her about her wedding and it turns out the woman is marrying the guy so he does not have to go to the frontlines of Vietnam, something that upsets her parents. The woman's fiancé, William (Elijah Wood), shows up and they spend time together. William makes sure she wants to go through with the wedding and tells her that it is just a piece of paper until the annulment goes through. The woman admits that she won't want an annulment and she in William sleep together. Meanwhile, a Czechoslovakian reporter, Lenka, keeps bugging Wade for an interview with Kennedy and when she finds out and tells him when the last time Czechoslovakia had free elections he finally gives in and gives her press credentials for the press conference after Kennedy's speech.
Meanwhile, a couple of an older man, Jack (martin Sheen) and his younger wife Samantha (Helen Hunt), check into the hotel. They talk about life and how the other is depressed and the husband then tells the wife that none of the things they have matter to him except for her.
Timmons tells Miriam about her husband's affair with phone operator. She confronts her husband and he goes and punches Timmons and tells him he can leave that day.
To wrap up: Tim Fallon leaves his wife, colliding with Sirhan Sirhan entering as he leaves. Diane and William get married happily. Virginia Fallon realizes she is unhappy with her life but performs that night. The drug dealer is arrested. Robert Kennedy arrives and John Casey greets and welcomes him. The one campaign worker meets with Robert Kennedy. Most of the characters converge at Kennedy's speech, with the kitchen workers talking about how crazy the kitchen is. Patricia flirts with Dwayne. Kennedy speaks and then leaves and everyone gets caught up in the moment.
Kennedy leaves through the kitchen and members of the crowd get separated. Finally in the kitchen they are all shaking Kennedy's hand when Sirhan Sirhan comes up to Kennedy and shoots him. People with Kennedy tackle Sirhan Sirhan, but the gun continues to go off. Both of the young workers (Shia LaBeouf and Brian Gerhaty), William, Samantha, and Timmons are all shot. Paul helps the Timmons. A waitress helps the two workers and the spouses cry over the others. Jose holds Kennedy's hand and gives him a rosary. The writing "The Once and Future King" is shown splattered with blood. A Kennedy speech about violence is played over the action. We are told by title cards that Kennedy died the next morning but all of the other victims survived.