A plucky kid named Don Juan grows up in Spain with two parents who don't get along. But his father dies before his mother can divorce the guy and Juan comes into a generous inheritance. In the meantime, his mom does everything she can to educate him herself and to protect him from the evils of the world—like knowing about sex. Try as she might, though, Don Juan inevitably feels some adolescent urges as he grows older. He also finds a way to satisfy these urges when he has an affair with a woman named Julia and leaves Spain entirely when Julia's husband tries to kill him.
Don Juan's journey away from Spain takes him onto a ship that wrecks in the middle of the Mediterranean ocean. He and some other men make it onto a lifeboat and DJ is eventually the only one who makes it to land—because the others mostly eat one another. DJ is discovered by a beautiful woman named Haidée and they fall in love. But Haidée's pirate father comes home one day and he sells Don Juan into slavery. (That's right—this poem has pirates.) Don Juan is then bought by a Turkish princess who wants him as a sex slave. But he escapes her when the Russian army attacks the Turks.
Now Don Juan has established himself as a Russian war hero after helping them fight the Turks. His new empress is Catherine the Great, and of course she falls in love with him at first sight. We don't know what Don Juan has to do to satisfy her, but we know that she likes it enough to give him all kinds of riches and to send him to England as some sort of ambassador.
Now that DJ is in England, Byron can finally start hammering away at how dumb he thinks English society is. He makes fun of all the fancy aristocrats that he meets. Eventually, the poem ends with Don Juan possibly having sex with yet another married, upper-class woman. The poem pretty much cuts off after that because Byron died of a fever before he could write anything more. To be fair though, the poem's pretty long as it is—over five thousand lines long.