ACT I. In the Courtyard of Ko-Ko's Official Residence in Titipu.
The act opens to find a group of Japanese nobles in characteristic attitudes. Nanki-Poo—still masquerading as a musician—returns to Titipu eagerly seeking Yum-Yum, as he has heard that Ko-Ko was condemned to death for flirting. He introduces himself, and, to his dismay, he learns from Pish-Tush that although Ko-Ko was indeed to have been beheaded, he was reprieved at the last moment and made Lord High Executioner instead. As the criminals must be executed in order, and Ko-Ko was next to be executed, no one else can be executed until Ko-Ko cuts off his own head.

For a small fee, Pooh-Bah, a ponderous aggregation of conflicts of interest, reveals that Yum-Yum is on her way home from school and will be wed to Ko-Ko this very afternoon.

The nobles herald the appearance of Ko-Ko and Ko-Ko appears and explains how he became the Lord High Executioner. He is full of ideas for his first official victim. He discusses with Pooh-Bah the plans for his forthcoming marriage.

A procession of school girls arrives, followed by Yum-Yum and her sisters, Pitti-Sing and Peep-Bo. Yum-Yum greets her betrothed decidedly less enthusiastically than she does Nanki-Poo. Ko-Ko introduces Pooh-Bah, who greets the girls very reluctantly. They respond teasingly.

Taking advantage of a few minutes alone with Yum-Yum, Nanki-Poo declares his love and shares with her the secret of his identity. Because of the excessive laws against flirting, Yum-Yum urges him to stay away from her. Nanki-Poo devises a plan to flirt by considering what they could do if it weren't for the law.

Ko-Ko, meanwhile, has received a letter from the Mikado, who is concerned that there have been no recent executions in Titipu and threatens severe repercussions if one does not take place within a month, including reducing the town to the rank of a village. Although, as Pooh-Bah points out, Ko-Ko is next in line for that honor, Ko-Ko understandably would prefer to find a substitute. Ko-Ko, Pooh-Bah, and Pish-Tush debate who should be executed.

He comes across Nanki-Poo, who is preparing to terminate his existence rather than face life without Yum-Yum, and the two men strike a bargain: Ko-Ko agrees to let Nanki-Poo marry Yum-Yum now, and, in return, Nanki-Poo agrees to let Ko-Ko behead him at the end of the month and marry his widow.
The townsmen want to know how Ko-Ko will comply with the Mikado's decree.  Ko-Ko introduces Nanki-Poo as his volunteer, and Nanki-Poo embraces Yum-Yum. All rejoice over this resolution, but the festivities are rudely interrupted by the appearance of Katisha. All cower except Pitti-Sing, who taunts Katisha. Katisha grieves her lost love. Furious at Nanki-Poo's rejection, she attempts to reveal his true identity. She is silenced by the crowd, but vows revenge.

ACT II.  Ko-Ko's Garden, Later the Same Day
As the act begins, Yum-Yum's sisters and friends are helping her make ready for her wedding. She concludes that she is more lovely than any other woman because she is a child of nature and takes after her mother.

Yum-Yum and her sisters are very sad at the thought of the very brief marriage, and Nanki-Poo enters and tries to lift their spirits. The wedding plans are disrupted upon Ko- Ko's discovery that, under the Mikado's law, when a married man is beheaded, his wife must be buried alive. Yum-Yum's enthusiasm for the marriage is suddenly diminished.

To spare Yum-Yum this grim fate, Nanki-Poo decides to kill himself at once. This, however, would leave Ko-Ko with nobody to behead—just as word arrives that the Mikado is at this very minute approaching Titipu. Nanki-Poo offers himself for immediate decapitation, but Ko-Ko is not equal to the task. Ko-Ko realizes, though, that he can accomplish the same purpose by swearing a false affidavit that he has done the deed, provided that Nanki-Poo leaves at once and never comes back. Since Nanki-Poo will not leave otherwise, Ko-Ko sends Yum-Yum with him, and the happy couple goes off to be married just as the Mikado enters the town.

As the Mikado enters, he describes how he, in a fatherly kind of way, governs each tribe and sect. Then he describes how he wants to make the punishment meet the crime.
The Mikado is delighted to hear that an execution has taken place, and is eager to hear the details. Ko-Ko, assisted by Pitti-Sing and Pooh-Bah, recounts a highly creative description of the execution.

The Mikado's visit, however, concerns another matter: at Katisha's prompting, he is seeking the whereabouts of his son. Unfortunately, this turns out to be Nanki-Poo, the man Ko-Ko has just testified he beheaded. Ko-Ko and his accomplices are declared guilty of "compassing the death of the Heir Apparent" and scheduled to die after luncheon — "something humorous, but lingering, with either boiling oil or melted lead."

The Mikado declares that he is sorry for them, but it is an unjust world, and virtue is triumphant only in theatrical performances. Their only hope is to admit the falsehood of the affidavit and of their testimony, and produce Nanki-Poo alive and well. Nanki-Poo, however, having already married Yum-Yum, is no longer free to marry Katisha and thus cannot reveal himself without risking both his own life and his wife's. Ko-Ko is left with no other choice but to woo, win, and wed Katisha herself.

Katisha is very upset over the loss of Nanki-Poo  Katisha at first refuses Ko-Ko, but he then sings her a touching ballad, and she accepts him.

Nanki-Poo then returns to life and presents himself—and his new bride—to his father. Ko-Ko devises an explanation of his subterfuge that satisfies the Mikado, who commutes his death sentence to one of life with Katisha. All ends with laughing song and merry dance.

--Courtesy of the Gilbert & Sullivan Society of Austin