Before the opera story begins, Nanki-Poo, the Mikado’s son, fled from his father’s palace to escape being compelled to marry Katisha, an elderly lady of the court; Nanki-Poo has become an itinerant minstrel who falls in love with Yum-Yum, however, he cannot marry her because Ko-Ko, her guardian, has decided to marry her himself. Nanki-Poo learns that Ko-Ko has been sentenced to death for violating the Mikado’s law against flirting. As Act I opens, Nanki-Poo has arrived in Titipu to determine if Ko-Ko has been executed, and, therefore, if Yum-Yum is free to marry him. He encounters Pooh-Bah, a corrupt public official, and Pish-Tush, a noble, who inform him that Ko-Ko was reprieved at the last moment by a set of curious chances, and then raised to the exalted rank of Lord High Executioner. Nanki-Poo turns to despair when he learns that Ko-Ko plans to marry Yum-Yum immediately. There have been no executions in Titipu since Ko-Ko became Lord High Executioner. Ko-Ko receives a letter from the Mikado ordering him to execute someone or else lose his post as Lord High Executioner. As Ko-Ko ponders his dilemma of trying to find someone to execute, Nanki-Poo appears, vowing suicide because he cannot marry Yum-Yum, the women he loves. Ko-Ko offers to allow Nanki-Poo to marry Yum-Yum for one month, after which, he will become his execution victim. Suddenly, Katisha appears and discovers Nanki-Poo, the vanished object of her love. After she is driven away, she rushes to inform the Mikado that his son has been found. As Act II opens, Yum-Yum prepares for her one-month marriage to Nanki-Poo. Ko-Ko arrives with the shocking revelation that he has discovered a law decreeing that when a married man is executed, his widow must be buried alive: under those horrible conditions, the marriage between Yum-Yum and Nanki-Poo is canceled. Nevertheless, Ko-Ko must find a “substitute” for execution or he will be decapitated by the Mikado. Nanki-Poo contrives a solution to save Ko-Ko’s life: a false affidavit confirming his own execution, but in exchange, he must be allowed to marry Yum-Yum and leave the country forever. Ko-Ko agrees. The Mikado arrives in Titipu. Ko-Ko believes that the purpose of his visit is to confirm that an execution has taken place so he produces the affidavit and proceeds to describe the execution with gusto. However, the Mikado has actually come to Titipu in search of his lost son and learns from the affidavit that Ko-Ko and his ministers executed his son; he declares them guilty of “composing the death of the Heir Apparent”; their only hope to avoid execution is to produce Nanki-Poo alive. Nanki-Poo hesitates to reveal himself, fearing that if Katisha learns that he has married Yum-Yum she will have him executed. The dilemma is resolved by Ko-Ko, who, at Nanki-Poo’s suggestion, woos, wins, and weds Katisha. All are reconciled as they celebrate Nanki-Poo’s marriage to Yum-Yum and Ko-Ko’s marriage to Katisha.