Act I

The issues of "Law and Order" and "Public Morality" it seems, are not new. Before our tale begins, the Mikado of Japan, much concerned with keeping the young men of his realm in the paths of righteousness, has decreed that "flirting" is a crime punishable by decapitation. In the town of Titipu, this edict has been greeted with a pardonable lack of enthusiasm. Resorting to "pilpul" not unworthy of our Jewish sages, the Elders of that town have appointed as Lord High Executioner one Ko-Ko, a tailor, the first to be condemned under the new law, on the grounds that he "cannot cut off another head until he's cut his own off." Too proud to serve under a commoner, all the noblemen have resigned their high offices. The "enormous swell" Pooh-Bah, explaining that his family pride is so overwhelming that he must mortify it continually (mostly by taking bribes), has assumed all their positions under the title of Lord High Everything Else.

As the curtain rises, we find the noble lords of Titipu practicing their 'court etiquette', no doubt because today Ko-Ko is going to celebrate his betrothal to his beautiful young ward, Yum-Yum. Enter Nanki-Poo, the son of the Mikado, but now in disguise as a "second trombone." He has fled his father's court to escape the amorous attentions of Katisha, an elderly noblewoman of formidable mien and waspish disposition, who claims him in marriage under his father's edict. He has fallen in love with Yum-Yum, and hearing of Ko-Ko's condemnation has come to woo her. After demonstrating to the nobles his versatility as a minstrel, he learns from Pish-Tush and Pooh-Bah the true state of affairs.

Ko-Ko makes an impressive entrance and tells us of his "little list of society offenders" none of whom would be missed. Actually, he's much too tender hearted to kill even a fly. Now we meet Yum-Yum and her equally captivating school chums, led by Pitti-Sing and Peep-Bo, but before the betrothal can take place awful news arrives. The Mikado demands an execution within a month, or he will ruin the town. Nanki-Poo has been contemplating suicide, but when Ko-Ko tries to persuade him to be executed in style instead, he lightheartedly agrees, in exchange for a month of marital bliss with the delicious Yum-Yum. Since Ko-Ko's only alternative is to try to chop off his own head, he reluctantly agrees.

General rejoicing greets Ko-Ko's announcement of the bargain they have struck, but it is disrupted by the arrival of Katisha to claim her "perjured lover." When her attempts at persuasion fail and her threats are rejected with contempt, she tries to reveal Nanki-Poo's true identity, but is shouted down. The curtain falls as she storms off in fury to seek vengeance from the Mikado, unheeded by the happy Titiputians.

Act II

The second act finds Yum-Yum perparing for her wedding, although her ecstasy is diminished by the thought of its short-lived nature. This melancholy is changed to downright horror when Ko-Ko reveals that protocol will require her, as Nanki-Poo's widow, to be buried alive! Not surprisingly, Yum-Yum's gossamer ardour is frayed to breaking-point when confronted with the cost of one's mouth's bliss. Delicately but firmly she convinces Nanki-Poo of the practical disadvantages of the scheme. At this moment the Mikado and his train are heard approaching. Ko-Ko jumps to the conclusion that the Mikado expects an execution to have taken place.

In a panic, he gets rid of the importunate lovers by arranging their marriage and flight, and decides that falsehood is the better part of valour. After the Mikado has sung of his desire to "make the punishment fit the crime," Ko-Ko, abetted by Pooh-Bah and Pitti-Sing, paints a grisly and colourful picture of the 'execution' for the Mikado's delectation – a delight which is slightly diminished by the discovery that the victim was his own son and heir; this shadow on the royal satisfaction is quickly banished by the guilty trio being condemned to a lingering, if humorous, death, after luncheon.

Disaster can only be averted by Nanki-Poo coming back to life. But the thwarted Katisha is bound to demand his death for marrying Yum-Yum, so Ko-Ko's only resort, to his dismay, is to woo, win and marry Katisha himself, and saves Titipu from disgrace. After the Mikado passes over a rather dubious explanation of the 'execution' so vividly described to him, a scene of general celebration brings down the curtain.