In a Cornish village a group of professional bridesmaids fret that there have been no weddings in the last six months and urge Rose Maybud, the prettiest maiden in the village, to move things along by marrying. Rose’s guardian, Dame Hannah, reveals that she is the victim of a doomed young romance. She has vowed to remain forever single since her lover revealed on their wedding day that he was one of the cursed Baronets of Ruddigore. She relates the legend of the curse: Centuries ago, the first Baronet of Ruddigore, Sir Rupert Murgatroyd, burnt a witch at the stake, who as she died cursed all future Baronets of Ruddigore to commit a crime every day or die in inconceivable agonies.
Rose reveals that she is fond of a shy young farmer, Robin Oakapple, but is constrained by Victorian etiquette from making the first move. (Complicating matters, Robin is in reality Sir Ruthven Murgatroyd, one of the cursed Baronets of Ruddigore. To avoid the curse, he has fled his ancestral home and assumed this new identity, leaving his unfortunate younger brother Despard Murgatroyd to take on the curse.)
Robin calls upon his best friend Richard Dauntless – a sailor and his foster brother – to court Rose for him. Overcome by Rose’s beauty, Richard sets about wooing her for himself and nearly succeeds in doing so until Robin manages to win her back.
Mad Margaret has been driven wild by her love for Sir Despard Murgatroyd. She is jealous of Rose, whom Despard intends to carry off, until Rose assures her she is pledged to another. Some eligible bachelors arrive from London, setting the bridesmaids’ hearts aflutter, before Despard appears and spoils their fun.
Richard, intent on winning back Rose, reveals to Despard his elder brother’s deception. In the middle of Robin and Rose’s wedding vows, Despard exposes Robin’s true identity – Sir Ruthven Murgatroyd – to the whole village. Despard, now a reformed man, marries Margaret, and Rose reluctantly rejects Robin for Richard. Chaos ensues.
Inside the ancestral castle of Ruddigore, Robin (now Ruthven) finds himself a very ineffectual bad baronet. He gives his reluctant blessing to Richard and Rose’s union before the ghost of Sir Roderic and all the other ancestors in the family portrait gallery awaken from their frames to give him a taste of the torturous death that awaits him if he does not obey the curse. Robin concedes and sends his valet Adam to go to the village and abduct a lady – “Any lady!”
Newlyweds Despard and Mad Margaret, now models of respectable piety, visit to urge Robin to renounce his life of crime. Adam soon returns with his abductee, Dame Hannah, who proves formidable indeed. Robin cries out for his uncle’s protection from her. Sir Roderic is amazed to come face to face with Dame Hannah, his “Little Nannikins”, the girl to whom he had once been engaged.
In a sudden epiphany, an idea occurs to Robin: A Baronet of Ruddigore who refuses to commit the daily crime must die and, therefore, to make such a refusal is tantamount to suicide; but suicide is a crime in itself! Consequently, Sir Roderic should never have died and the curse should not have been handed on. By this logic Robin and indeed all the Murgatroyds are free of the curse! Rose happily resumes her engagement to Robin. Sir Roderic and Dame Hannah reunite, and all ends happily.