The Mikado has often been described as Gilbert & Sullivan's most popular opera - if not the most popular opera ever to be written! The play begins in a little town called Titipu, where Nanki-Poo, disguised as a minstrel, is looking for Yum-Yum, the ward of Ko-Ko. Nanki-Poo tells Pish-Tush, a noble lord, that he had met and fallen in love with Yom-Yum a year ago when he was a member of the town music group.
Yum-Yum was given in marriage to her guardian, Ko-Ko. Nanki-Poo had heard that Ko-Ko had been condemned to death for flirting, and was returning to claim Yum-Yum's hand in marriage. Regrettably he learns that Ko-Ko had been let out on bail, and appointed Lord High Executioner. The situation worsens when Pooh-Bah, who holds every major office in town, informs Nanki-Poo that Yum-Yum and Ko-Ko are to be married that very day!
Yum-Yum and her two sisters, Pitti-Sing and Peep-Bo appear with a group of their friends from school. Nanki-Poo reveals to Yum-Yum that he is in fact the son of the Mikado, and she admits that she does not love her guardian, whom she is supposed to marry. The two lovers realize that their situation is bleak, and Yum-Yum leaves Nanki-Poo, who then attempts to kill himself.
Meanwhile Ko-Ko has received a letter from the Mikado, who demands that a beheading has to take place in Titipu within the next month, or else Titipu would be reduced to the rank of a village, and the post of the Lord High Executioner would be abolished. Ko-Ko comes across Nanki-Poo trying to kill himself, comments that committing suicide is a capital offense, and offers to do the job "professionally". Nanki-Poo agrees, on condition that he can marry Yum-Yum, and enjoy married bliss for a month before being beheaded. After the execution, Ko-Ko would be able to marry the widowed Yum-Yum. However, Katisha, who is madly in love with Nanki-Poo, arrives on the scene during the celebrations, and threatens to reveal his true identity. She is shouted down, and the celebrations continue.
The second act starts out with Yum-Yum preparing for her wedding, but then she finds out that by law the widow of a beheaded man must be buried alive. Nanki-Poo now faces a dilemma, because if he holds Yum-Yum to the marriage, she will die a hideous death. However, if he allows her to go, she has to marry Ko-Ko without further delay. The marriage is put off, and Nanki-Poo wants Ko-Ko to kill him there and then. But Ko-Ko is unable to kill him, and to make matters worse, the Mikado is due to arrive in town within just a few minutes. In desperation, Ko-Ko arranges for an affidavit of Nanki-Poo's execution.
The Mikado arrives with Katisha, who makes a big to-do of being his daughter-in-law elect. Ko-Ko presents the certificate of execution, just to be told that he had "executed" the heir to the throne of Japan! A desperate Ko-Ko and Pooh Bah find Nanki-Poo, and beg him to present himself, thus absolving them from his death. But Nanki-Poo, now married to Yum-Yum is afraid of Katisha's rage, and is not willing to do this unless Ko-Ko agrees to marry Katisha himself. Meanwhile, Katisha is mourning the death of Nanki-Poo, and is at first reluctant when Ko-Ko tries to woo her. However, he eventually wins her over with a sad, unhappy song - Willow, Tit-Willow.
Nanki-Poo returns and presents himself, and his new bride, to his father. Ko-Ko puts together an explanation of his deception that satisfies the Mikado, who commutes his death sentence to one of life with Katisha. The "threatened cloud has passed away", and story ends happily for all with "laughing song and merry dance".
