Bill & Ted are rehearsing for Wyld Stallyns to play at the school dance (no one else tried out). They arrive late at school and accidentally slip into the girls' health class during a movie and are caught by Miss Spleen, who reprimands them for being late for her math class and forgetting homework and threatens to have them expelled and kept from playing at the school dance. Later at the Kozy Korner, Ted purchases a blue Frosty Slush in a silver collector's cup before the dudes head back in time to pick up the Princess Babes to sing with the band at the dance. They accidentally end up in the middle of the Crusades where they run afoul of Saladin. He is ready to arrest them until he spots Ted's silver collector's cup and believes it to be the Silver Chalice. Just then the Black Knight bursts in and steals the Silver Chalice for himself. Believing Bill & Ted crossed them, Saladin's men start after them but they are able to escape in the booth. They arrive in England just as the Black Knight returns from the Crusades with the Silver Chalice as a gift for the king, who offers the Knight one of his daughters in marriage as a reward. Determined to save the babes, Bill & Ted infiltrate the castle and dress as jesters to entertain at a banquet. Their feeble attempts at juggling and magic acts tips off the Black Knight, who announces they are there to steal the Silver Chalice. The dudes manage to get away and sneak into the rooms of the Princess babes, where the Black Knight shows up and demands Joanna's hand in marriage. At the wedding, the princesses are saved when Rufus disguises himself as Joanna long enough to allow the dudes and babes to escape. Chasing after them, the Black Knight is sucked into the time circuit and swept along to San Dimas with them. At Ted's house the Black Knight lands on the roof and crashes down through the chimney. They keep him in a state of unconsciousness by playing Lawrence Welk polka classics for him. Bill & Ted leave Deacon with the Princess Babes and Black Knight as they hurry to school to avoid being punished by Miss Spleen. Back at Ted's house, the Black Knight comes to and chases after Deacon and the Princesses. Unable to reach anyone by phone, Bill & Ted hurry home to find out what's happened. They spot Detective Logan racing to where the Black Knight is riding through town, causing havoc. Realizing they must do something, the dudes look in the Circuits of Time Directory for someone to help them and come across an ad for Wyatt Earp. They travel back to Tombstone, Arizona, where they land on a stagecoach Jesse James is trying to rob. They continue on to town with Jesse James on their tail, leading the bandit right into the hands of sheriff Wyatt Earp. Wyatt Earp declines going back to San Dimas with them, saying he has a lunch date with Calamity Jane, but deputizes the dudes to take on the Black Knight themselves. Returning to San Dimas, they face the Black Knight in the center of town at high noon and they attempt to rope the Black Knight but fail (Ted forgets to hang on to his end of the rope). The Black Knight escapes, and Bill & Ted are picked up and returned to school by Ted's dad. They are just in time for Miss Spleen's math class where they haven't done their homework. Miss Spleen gives them a chance to still attend the dance by asking them to solve a long, complicated problem on the blackboard. They take a guess and actually get it right, so they are allowed to play at the dance, only to find out Miss Spleen will be the chaperon. At the dance, Wyld Stallyns are performing when the Black Knight bursts in. He is knocked off his horse by the loud cacophony of sound when Joanna accidentally knocks Ted's electric guitar out of his hands, and when Miss Spleen investigates she and the Black Knight fall immediately in love. Rufus arrives to whisk the happy couple away back to England, and Wyld Stallyns continues to play at the dance.