When he was young, Jack Carstares took the blame when his younger brother was caught cheating at cards. He was ostracized from society and fled to the Continent, where he eventually made his fortune gambling and teaching fencing. Now he has returned to England, where he plays at being a highwayman (but in fact, gives all his ill-gotten gains to the poor). When his younger brother realizes that Jack is back, he is wracked with guilt, but as before his love for the spoiled Lavinia keeps him from revealing the truth.
Meanwhile, the Duke of Andover (called Devil by friends and foes alike) has fallen for young Diana. He woos her under a false name, then tries to abduct her--but Jack, in his guise as a highwayman, fights him back. The Duke returns to London to come up with another plan to get Diana into his clutches, and Jack and Diana fall in love while she nurses him from his wounds. But Jack is still unwilling to cast aside his bad reputation, and so he and Diana cannot get married. Alack! It all ends happily, of course.
This is a very odd book. First off, the morality system is quite strange: when Jack is accused of cheating at cards his friends and family cast him out entirely. But the Duke can try to seduce, abduct, and full on rape a lady *repeatedly*, and no one considers so much as disinviting him from a party. Nor, in fact, will Jack even reveal the Duke's real name to the woman he tried to ruin--he puts actual effort into keeping the Duke's true identity a secret. Madness!
The other oddity is that the male friendships are given so much more intensity and page time than the romance between Jack and Diana. Jack and his valet Jim have an adorably/uncomfortably (depending on your feelings about class differences) feudal relationship. A great deal of the book is made up of conversations between Jack and O'Hara, who adore each other, banter constantly, and defend each other viciously. And, of course, there is the Duke and his bff Fortescue. The book opens with a letter from the Duke to Fortescue, and finishes with the Duke and Fortescue talking in their shared lodging in Venice. The Duke says Fortescue is literally the only person he likes in the whole world, and Fortescue spends all his time hanging out with the Duke, trying to get him to talk about his feelings.
Although Diana does not get nearly the amount of page time the menfolk do (and in fact, Lavinia and O'Hara's wife get about as much page time as she does), she's a good character. She particularly shines in her final showdown with the Duke. But as for the Duke himself--I don't get why Heyer would want to write this character again, nor why anyone finds him fascinating. He's just another drawling menacing asshole, another Marquis St. Evrémonde or Lucius Malfoy. Maybe I'm just tired of the type, which seems to be everywhere in Regency romances.