Rome, AD 40–41. Claudius is living with the ex-prostitute Calpurnia in meager circumstances. Caligula has turned the palace into a brothel where he sells the wives of high-ranking Senate members to the highest bidder during sexual orgies and forces Claudius to take money at the door. As a joke, he arranges for Claudius to marry the much younger, extremely beautiful Messalina. Totally insane, Caligula makes his horse Incitatus a senator, and takes his legions on a campaign to Germany to put down an alleged rebellion and then to the English Channel where he attempts to do battle with Neptune, bringing back seashells as booty. Returning to Rome, he decides to execute the entire Senate for not awarding him a Triumph for his 'victory,' but his wife Caesonia and Claudius persuade him not to. Cassius Chaerea, a leading Praetorian officer whom the Emperor continuously mocks, forms a plan with several others to assassinate Caligula. They strike during the games held to celebrate Augustus, luring him away from his German Guards and killing him. Cassius proceeds to murder Caligula's wife Caesonia, and their infant daughter Julia Drusilla, attempting to wipe out the Imperial family once and for all. While the suddenly- leaderless Praetorian Guard are looting the palace, they come upon Claudius, hiding behind a curtain, and proclaim him Emperor over his own protestations.