Venetian morality guardians burst into the apartments of notorious libertine Giovanni Jacopo Casanova. He is indicted for atheism, possession of indecent literature, and fornication – the last a hard charge to shake off, as he happens to be in bed with an equally indecent woman. As his friend Senator Bragadin tries to intercede on his behalf, the imprisoned Casanova is tormented both by wicked jailer Lorenzo and by memories of his lost love, the innocent country girl Cristina, whom he betrayed.

Bragadin uses his influence to have Casanova moved to a larger, better-lit cell — just as he was making headway on an escape tunnel. Discovering the hole, Lorenzo takes the opportunity to strip search Casanova, on pretext of looking for the instrument he dug it with. To increase Casanova's misery, he has to share his new cell with an acquaintance, the obnoxious broker Schalon, whose perfumed silks bring with them memories of happier, pre-incarceration times.

Continued incarceration and the conversation of his cellmate are starting to take toll on Casanova. His mind is cast back to a visit he once made to Mantua. On the run from an angry mob after he disrupted a commedia dell'arte performance, he takes refuge in the home of an elderly judge and his beautiful daughter. Seeing the old man's credulity about holy relics and the occult, Casanova hatches a plot to part him from his money and the girl from her virginity.

The act of impulsively throwing a stone through a window sparks a chain of memories from Casanova. He reflects on his Venetian prison experiences as he witnesses the torture and execution of Robert-François Damiens, would-be assassin of Louis XV, from the window of a Parisian apartment. Later, in Grenoble, his memories of both occasions haunt him as he's drawn to a pious young woman he sees at a Vivaldi concert and seduces her compulsively despite his friend Valenglart's warnings.

On a journey to London, Casanova meets an Englishman so thick he may as well be Lorenzo's double, and reunites with Schalon, whose pomposity has only increased since his release. Finding that English women are immune to his charms, Casanova advertises for a female tenant with intent of seducing her, but the plan sours when he finds himself genuinely attracted to her. Obsessed with and repulsed by sex, Casanova wonders whether he's any freer than he was in prison.

Casanova longed for books during imprisonment; now, as an old man, he is surrounded by them. Aged 73, he works as a librarian in a German castle, although disquieting rumours have reached the ears of tyrannical major-domo Herr Feldkirchner about the whereabouts of Casanova's hand in relation to the cook's skirts. Plagued by a heart ailment, Casanova struggles to put to text form the memories of his life, including his triumphant prison escape.