Plot
Caligula is the young heir to the throne of his great-uncle, Emperor Tiberius. After a blackbird flies into his room, interpreted by Caligula as a bad omen, a head of the Praetorian Guard, Naevius Sutorius Macro, tells Caligula that Tiberius has summoned him to Capri where the Emperor lives with his close friend Nerva, Caligula's dim-witted uncle Claudius, and Caligula's cousin (Tiberius's grandson) Gemellus. Caligula fears assassination but his sister and lover Drusilla persuades him to go.
Caligula finds that Tiberius is showing signs of advanced venereal diseases and has become embittered with Rome and politics. The depraved Tiberius swims with naked youths and watches degrading sex shows involving deformed people and animals. Caligula observes with fascination and horror. Tensions rise when Tiberius tries to poison Caligula in front of Gemellus. Nerva commits suicide and Caligula tries to kill Tiberius but loses his nerve. Macro kills Tiberius to prove his loyalty to Caligula, witnessed by Gemellus.
Caligula is proclaimed the new Emperor and proclaims Drusilla as his equal, to the apparent disgust of the Roman Senate. Drusilla, fearful of Macro's influence, persuades Caligula to get rid of him. Gemellus testifies at a mock trial that Macro murdered Tiberius, then has Macro's wife Ennia banished from Rome. After Macro is executed in a gruesome public game, Caligula appoints Tiberius's former adviser Longinus as his personal assistant while pronouncing the docile Senator Chaerea as the new head of the Praetorian Guard.
Drusilla tries to find Caligula a wife among the priestesses of the goddess Isis, the cult they secretly practice. Drusilla rejects Caligula's marriage offers because they are siblings and he marries Caesonia, a priestess and notorious courtesan, after she bears him an heir. Caligula is popular with the masses but the Senate disapproves of his eccentricities. Caligula's true personality emerges when he jealously rapes both a virgin bride and her husband on their wedding day and orders Gemellus's execution to provoke a reaction from Drusilla.
After discovering that Caesonia is pregnant, Caligula develops a severe fever. Drusilla nurses him back to health and Caesonia bears him a daughter, Julia Drusilla. During the celebration, Drusilla collapses with the same fever he had. Soon afterward, Caligula receives another ill omen in the form of a blackbird. Despite praying to Isis out of desperation, Drusilla dies and Caligula has a nervous breakdown.
Caligula walks through Rome in a deep depression disguised as a beggar and is briefly jailed for causing a disturbance after watching an amateur performance mocking his relationship with Drusilla. Caligula proclaims himself a god and vows to destroy the senatorial class he loathes. Senators' wives are forced to work as prostitutes, estates are confiscated, the old religion is desecrated, and the army embarks on a mock invasion of Britain. Unable to tolerate his actions, Longinus conspires with Chaerea to assassinate Caligula.
Another blackbird appears in Caligula's bedroom but only Caesonia is frightened of it. Claudius becomes suspicious of Chaerea and follows him to a play where Caligula and his family are attacked in a coup led by Longinus and Chaerea. Caesonia and Julia are murdered, and Chaerea stabs Caligula in the stomach. With his final breath, the Emperor defiantly declares "I live!" as his and his family's bodies are thrown down the stadium's stairs. A horrified Claudius is proclaimed Emperor by the Praetorian Guard. The people of Rome, accustomed to chaos, clean the blood off the marble floors. The film ends with Caligula's horse, Incitatus, discovering the dead bodies of Caligula and his family.
