The Bridesmaids suggest to Dame Hannah, Rose's aunt, that she might take a husband, but she explains that she is pledged to an eternal maidenhood. Long ago she was engaged to Sir Roderic Murgatroyd, the bad baronet of Ruddigore, but, when she discovered who he was, she left him. She tells how the witch's curse was put on the house of Ruddigore which obliges the holder of the title to commit a crime every day or die in agony.

Rose Maybud describes the difficulty of attracting suitors without violating the principles f etiquette to which she is devoted. She loves Robin Oakapple, a young farmer, but he is as sky as she is modest, and the pair find it impossible to tell each other of their love. It is discovered that Robin is living in Rederring in disguise. His true identity - Sir Ruthven Murgatroyd, rightful heir to the Ruddigore baronetcy - is revealed by his faithful servant, Old Adam. The responsibility for the daily crime has fallen to his younger brother Despard, who believes Ruthven dead.

Richard Dauntless, a Man-o'-war's man and Robin's foster brother, arrives in Rederring after ten long years at sea. He is a burlesque of the braggart sailor tradition and offers to woo Rose on Robin's behalf - an undertaking in which he is so successful that he wins her for himself. However, when Rose discovers that Robin does in fact love her, she decides to marry him after all.

Mad Margaret enter. She loves Sir Despard but has been driven mad by his villainy and indifference towards her. Despard, followed by his "crew," arrived in Rederring and Richard Dauntless, acting on the "dictates of his heart," reveals the secret of Robin's true identity to Sir Despard. The result is that when the villagers assemble for Rose and Robin's marriage Despard denounces Robin as the true baronet of Ruddigore, and Rose is compelled to return to Richard. Despard, "a virtuous person now" marries Margaret; Robin is saddled with the title and the attendant curse.

On the walls are portraits of the earlier baronets. Robin, now Sir Ruthven, is consulting his servant, Old Adam - the "confidential advisor to the greatest villain unhung!' They are interrupted by the arrival of Rose and Richard, who have come to ask Ruthven's consent to their marriage - which he eventually gives.

The stage darkens and the Murgatroyd ancestors step down from their frames. Sir Roderic, the twenty-first baronet, asks Ruthven about his daily crimes and professes himself dissatisfied with their trivial nature. He demands that a lady shall be carried off that very day. Ruthven is reluctant to do such a thing but consents when given a taste of the death agonies by the ancestors. The ancestors return to their frames and Old Adam is dispatched to carry off a lady. Ruthven, in a little known song, sings of the crimes he will perpetrate and complains that "A baronet's rank is exceedingly nice, but the title 's uncommonly dear at the price."

Despard and Margaret enter. They are no totally respectable and rule a National School. They exhort Ruthven to abandon his career of crime, even at the cost of his life. He agrees, but a complication occurs when Old Adam returns having, as instructed, carried of a lady. She turns out the be Dame Hannah, who becomes so ferocious that she chases Ruthven with a large sword; he calls upon Sir Roderic to protect him. The latter steps down from his frame and he and Hannah immediately recognize each other as lover of long ago.

Ruthven returns in great excitement, an idea having just occurred to him. A baronet of Ruddigore, he explains, incurs death by failing to commit his daily crime; therefore, to refuse to commit his daily crime is tantamount to suicide; but suicide is itself a crime. From this it follows that neither Sir Roderic nor any of the ancestors ought ever to have died. They all once more descend from their frames and resume their existence. Roderic is unit to Dame Hannah; Ruthven is now free to marry Rose; Richard Dauntless and Old Adam have to be content with a professional bridesmaid each. Led by Margaret and Despard they all toddle off to settle in the town of Basingstoke.