The tale is set in the town of Titipu. The noble rulers of the town are headed by Ko-Ko, the Lord High Executioner, and Pooh-Bah, the Lord High Everything Else! Nanki-Poo. the son of the Mikado, comes to Titipu, disguised as a wandering minstrel, in search of Yum-Yum, one of Ko-Ko's wards. He learns that Yum-Yum is returning from school in order to be married to Ko-Ko. After a scene in which the "three little maids" meet Ko-Ko, Pooh-Bah and Nanki-Poo, Yum-Yum and Nanki-Poo are left alone together. They confess their love for each other. Nanki-Poo confesses his real identity and explains that he had to flee from the court to evade the attentions of Katisha.
Ko-Ko receives warning that the Mikado is concerned because there have been no executions in Titipu: he looks around for a victim, and, discovering Nanki-Poo on the verge of suicide, arranges for him to marry Yum-Yum upon condition that a month later Nanki-Poo will be executed and then Ko-Ko will marry Yum-Yum.
A party follows: but it is rudely interrupted by Katisha, who arrives in search of Nanki-Poo and who tries to tell the crowd who he really isan attempt which is drowned by the cries of the people.
Act II is set in Ko-Ko's garden where Yum-Yum is preparing
for her marriage. Her joy is turned to dismay when she learns that if
her husband is executed, she must be buried alive!