Act I - Titipu town centre, Ko-Ko's house and garden

A chorus of Titiputians is going about the business of the day. They explain who they are and what they are doing.

Enter Nanki-Poo. He asks for Yum-Yum, and the men demand to know who he is. Nanki-Poo declares himself a wandering minstrel. He gives samples of a pathetic lament and a patriotic song- the chorus joining in.

Nanki-Poo explains that he loves Yum-Yum. He had abandoned his courtship on learning that she was to marry Ko-Ko, 'a cheap tailor,' but he has returned, hearing that Ko-Ko had been condemned to death for flirting. Push-Tush explains that Ko-Ko was indeed under sentence of death, but is so no longer; he has been made Lord High Executioner. The Mikado had made flirting a capital crime. To get round the law, Ko-Ko was made executioner, because he couldn't execute anyone else without first executing himself!

Enter Pooh-Bah. He is the archetypical smooth politician, charmingly corrupt. Nanki-Poo gives him money and Pohh-Bah tells him about Ko-Ko and Yum-Yum. Nanki-Poo's suit is hopeless, for Yum-Yum is to wed Ko-Ko today.

"Behold the Lord High Executioner." Ko-Ko enters surrounded by crowds of admirers. He tells them how he was promoted.

Very happy with his reception, he lists all the people whom society could do without. The Lord High Executioner expounds his 'little list' of potential victims and "they'd none of them be missed!"

Ko-Ko discovers that if he wants the city to pay for his wedding Pooh-Bah will have to be handsomely bribed. The chorus of girls now enters, heralding Yum-Yum and her two friends, Peep-Bo and Pitti-Sing. Fresh from school, they wonder "what the world can be."

Of the "three little maids from school" Yum-Yum is already engaged, and the other two do not expect to have to wait very long.

Ko-Ko, entering with Pooh-Bah, kisses a reluctant Yum-Yum. Nanki-Poo comes in and is enthusiastically welcomed by the girls but is removed by Pish-Tush. The girls laugh at Pooh-Bah behind his back. The girls have more high-spiritedness than etiquette, but Pooh-Bah clings to the prerogative of rank.

Nanki-Poo comes back and confesses to Yum-Yum that he is really the son of the Mikado, in disguise. The two are in love, but what can they do, since Yum-Yum is engaged to Ko-Ko?

Ko-Ko receives a letter from the Mikado, complaining that no one in Titipu has been executed for a year. Someone must be executed within a month. Ko-Ko finds good reason for not beheading himself, and Pooh-Bah and Pish-Tush are disinclined to act as substitute-victims.

Nanki-Poo is about to hang himself for love, but he consents to allow himself to be executed in a month's time if he many marry Yum-Yum now. Reluctantly, Ko-Ko agrees.

The chorus of men and women learn that Ko-Ko has found a substitute. There is rejoicing and congratulations. But suddenly an evil-looking figure enters: "Your revels cease!" It is Katish, who claims that Nanki-Poo is engaged to her. She intends to disclose his true identity - but the others drown her out. Katisha vows vengeance, but everyone else ignores her in their happiness at the prospective wedding.

Act II - Later the same day

Yum-Yum is being prepared for her wedding by the other girls.

Art and nature combined to make a pretty bride, and maiden modesty enhances the effect.

Just as Nature rejoices in her loveliness, so (says Yum-Yum) "I am a child of Nature, and take after my mother." Yum-Yum compares herself to the sun and the moon. But when Nanki-Poo enters he finds Yum-Yum in tears, because in a month he is to be beheaded. Still, they (with Pitti-Sing and Pish-Tush) try to summon up the joy appropriate to a wedding morning.

Ko-Ko, entering, announces that he has discovered that when a man is beheaded, his wife must by law be buried alive. Yum-Yum does not relish this fate. Nanki-Poo, his marriage plans in ruins, considers it "a pretty mess."

The Mikado's impending arrival is announced. Ko-Ko has not yet executed anyone - and even when Nanki-Poo, now dejected, offers himself, Ko-Ko cannot bring himself to do the deed. So he decides to pretend that Nanki-Poo has been executed. Pooh-Bah is ready to be witness (for a fee). Ko-Ko sends Nanki-Poo and Yum-Yum away to get married to keep them out of sigh, and awaits the Mikado.

Entrance of the Mikado and Katisha. The chorus (in Japanese) welcomes the Mikado. He sings of his power, but Katisha, his "daughter-in-law elect" speaks for herself.

The Mikado describes his plans to "let the punishment firt the crime." Ko-Ko tells the Mikado of the 'execution,' with elaborate details. The Mikado, intrigued, demands a description of the event. Pitti-Sing, Ko-Ko and Pooh-Bah tell the Mikado how it all took place, with a wealth of corroborative detail.

But the Mikado has really come to discover where Nanki-Poo is. Horrors, his name is on the death certificate! The punishment for causing the death of the heir-apparent is terrifying. Of course the trio of criminals in this case could not be expected to have known what they were doing, and the Mikado is sorry for them; but the law is the law.

Ko-Ko, Pooh-Bah and Pitti-Sing angrily criticise each other. Nanki-Poo is about to leave on his honeymoon and declines to "come to life again" unless Ko-Ko can persuade Katisha to marry him - she will then have no further claim on Nanki-Poo.

Katisha enters and mourns for the loss of her love. Ko-Ko desperately courts Katisha. To persuade her he tells a sad story: A little bird once died of a broken heart, and so will Ko-Ko if Katisha rejects him.

Katisha accepts Ko-Ko. Since Ko-Ko declares he shares Katisha's somewhat bloodthirsty taste, they are evidently suited.

The Mikado enters for the execution. Katisha begs for the lives of Ko-Ko (now her husband), Pitti-Sing and Pooh-Bah. Senstaionally, Nanki-Poo now enters with Yum-Yum. The false execution is excused because, of course, "When your Majesty says 'Let a thing be done' it's as good as done." The Mikado is satisfied and all ends happily. In the end even Katisha is reconciled and all rejoice.