ACT 1

The show opens in the town of Titipu where the chorus of nobles are joined by Nanki-Poo, disguised as a minstrel, who is looking for Yum-Yum, the ward of Ko-Ko. A noble lord named Pish-Tush asks what Nanki-Poo's business is with Yum-Yum, and learns that the minstrel had seen the girl a year ago when he was a member of the town band and they had fallen in love, but Yum-Yum was betrothed to her guardian Ko-Ko, 'a cheap tailor.'

However on learning that Ko-Ko was condemned to death for flirting, the minstrel has hurried back to try to claim Yum-Yum. Pish-Tush tells him that far from being dead, Ko-Ko has in fact been let out on bail, and appointed Lord High Executioner! There is worse to come as Pooh-Bah, a nobleman who holds every major office of state, informs Nanki-Poo that Yum-Yum and Ko-Ko are to be married that very day.

Yum-Yum and her sisters, Pitti-Sing and Peep-Bo, appear with their schoolfellows and when she meets up with Nanki-Poo he reveals that he is actually the son of the Mikado.  When they are alone, Yum-Yum admits she does not love her guardian, but  the two lovers realise that their cause is hopeless and Yum-Yum leaves Nanki-Poo who then tries to kill himself.

Meanwhile, Ko-Ko has received a letter from the Mikado, who threatens to abolish the post of Lord High Executioner and reduce Titipu to the rank of a village unless a beheading takes place within a month. On seeing Nanki-Poo about to 'terminate an unendurable existence', Ko-Ko points out that suicide is a capital offence and offers to do the job professionally. Nanki-Poo agrees, on the condition that he can Marry Yum-Yum and enjoy one month of married life before he is beheaded.  After the execution Ko-Ko will then be able to marry the widowed Yum-Yum.

Amidst the celebrations, in storms Katisha - an imposing lady of the Mikado’s court - having tracked down the object of her affections, Nanki-Poo, and threatens to reveal his true identity.  She is outshouted by the town’s people who are determined to have a good time.

ACT 2

Act two opens with Yum-Yum being prepared for her wedding. But soon the awful fact is out that under the Mikado's law the widow of a beheaded man must be buried alive with him. This places Nanki-Poo in a dilemma, if he holds Yum-Yum to this marriage, she dies a hideous death, and if he releases her she must marry Ko-Ko at once.  The marriage is off, and Nanki-Poo determines to do away with himself that afternoon unless Ko-Ko will kill him at once.  But it turns out that Ko-Ko can't kill anything. To make matters worse, the Mikado and his suite are approaching the town and will arrive in ten minutes.  In desperation Ko-Ko arranges to draw up an affidavit of Nanki-Poo's execution.

The Mikado arrives with Katisha who makes much of being his daughter-in-law elect.  Ko-Ko presents his certificate of execution and he, Pooh-Bah and Pitti-Sing describe the event in exaggerated detail.  However when the Mikado reads it,  he sees that they have beheaded the heir to the throne of Japan!  Nanki-Poo is begged to present himself, alive, to his father, but now married to Yum-Yum, he is afraid of Katisha's wrath and declares that unless Ko-Ko will agree to marry the old hag himself, he and Yum-Yum will leave on their honeymoon at once.  Katisha, meanwhile, is mourning the death of Nanki-Poo and when Ko-Ko tries to woo her she is at first reluctant, but he wins the formidable lady with a pack of flattering lies and a sad, lovelorn song.

Katisha adds her powerful pleas to the Mikado for everyone to be pardoned. The Mikado, a bit bewildered by it all nonetheless pronounces that 'Nothing could possibly be more satisfactory!'