The Emperor of Rome, Caligula, has fallen madly in love with an unknown queen of great beauty, Teosena. She has come to ask for his help after the death at sea of her husband, Tigrane, King of Mauretania. Caligula orders a portrait of the beauty to be painted. This decision provokes the jealous wrath of his wife, the Empress Cesonia. It also plunges the slave painter to whom Caligula assigns the task into disarray – for he is none other than Tigrane, who survived the shipwreck and has disguised himself in order to get closer to Teosena! Rejected by her imperial husband, who is still vainly attempting to seduce the exiled queen, Cesonia gives him a potion to drink. Meanwhile, Tigrane, still disguised as the painter, tries to make himself known to his wife. But he must also reckon with the master who bought him as a slave, Artabano, King of the Parthians. He too has fallen in love with Teosena, and tries to kidnap her! In a magisterial mad scene, Caligula loses his reason. He disowns his wife, chases away his guests, and even woos the old nurse – no shrinking violet, though initially amazed at this honour – as he takes himself first for Hercules pursuing Diana, then for a shepherd in love with the moon. Faced with this alarming behaviour, the Senate makes an extreme decision. It deposes Caligula and installs the patrician Claudio in his place. Cesonia is recalled from her exile while Tigrane, who no longer doubts Teosena’s loyalty, seeks to assert his rights against his rival Artabano, who does not understand how a queen can prefer his slave to him. Then comes a sudden coup de théâtre: Caligula, earlier given up for dead after stabbing himself, regains his wits, and at the same time his love for his legitimate spouse! Everything then returns to normal, and the Emperor even helps Tigrane to regain the throne of Mauretania.