The story may at first glance seem merely frivolous — the obscene nonsense of schoolboys. It is a parody of Shakespeare's Macbeth with bits of Hamlet and King Lear tossed in.

As the play begins, Ubu leads a revolution, and kills the King of Poland and most of the royal family. The Queen of Poland then dies. The ghost of the dead king calls for revenge, prompting Ubu to begin killing the population and taking their money. Ubu's henchman gets thrown into prison; he then escapes to Russia, where he gets the Tsar to declare war on Ubu. As Ubu heads out to confront the invading Russians, his wife tries to steal money that Ubu has stashed in the palace. She is driven away by Bougrelas, the crown prince, who is leading a revolt of the people against Ubu. She runs away to her husband, Ubu, who has, in the meantime, defeated the Russians, and who has also been attacked by a bear. Ubu's wife pretends to be the angel Gabriel, in order to try to scare Ubu into forgiving her for her attempt to steal from him. They fight, and she is rescued by the entrance of Bougrelas, who is after Ubu. Ubu knocks down the attackers with the body of the dead bear, after which he and his wife flee to France, which ends the play.

The action contains motifs found in the plays of Shakespeare: a king's murder and a scheming wife from Macbeth, the ghost from Hamlet, Fortinbras' revolt from Hamlet, the reneging of Buckingham's reward from Richard III, and the pursuing bear from The Winter's Tale. It also includes other cultural references, for example, to Sophocles' Oedipus Rex (Œdipe Roi in French) in the play's title. Ubu Roi is seen to have been preceded in the spirit of outrageousness, and comic grotesquery by the great French Renaissance author François Rabelais's Gargantua and Pantagruel novels.

The language of the play is a unique mix of slang from the playground, code-words, puns and near-gutter vocabulary, set to strange speech patterns.