In the town of Titipu, Nanki-Poo, disguised as a second trombone, is looking for Yum-Yum, the ward and fiancee of Ko-Ko, the Lord High Executioner. When Yum-Yum finally catches sight of Nanki-Poo he reveals that he is the son of the Mikado, and when they are alone, she admits she does not love her guardian. 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 demanding a beheading takes place within a month. Nanki-Poo agrees to be beheaded, on the condition that he can marry Yum-Yum and enjoy one month of married life before his execution. Amidst the celebrations in storms Katisha, having tracked down the object of her affections, Nanki-Poo, and threatens to reveal his true identity.

Yum-Yum prepares 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.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. Ko-Ko presents the certificate of execution and The Mikado realises that they have beheaded the heir to the throne of Japan! Much debate and haggling ensues to try to save the lives of all involved leading to an unlikely marriage but the complete satisfaction of the Mikado. 

synopsis taken from The Gilbert and Sullivan Archive