The lights rise on a traditional Japanese scene. Gilbert and Sullivan look on from their clouds in heaven delighted that audiences are still enjoying their Mikado 100 years on, though suggesting some updating is needed to make it a bit more ‘hip’.
The scene changes to the neon town of Titipu, where the gentlemen of Japan are interrupted by the entrance of Nanki-Poo. The second trumpeter demands to know where to find his love – Yum-Yum, the beautiful ward of the tailor Ko-Ko. Their relationship had been doomed as she was engaged to her guardian but Nanki-Poo has recently heard the happy news that Ko-Ko was condemned to death by the almighty Mikado for breaking the law against flirting. And so Nanki Poo has rushed to Titipu in order to marry Yum-Yum.
He learns that Ko-Ko has since been freed and made Lord High Executioner. He also hears that Yum-Yum and Ko-Ko are slated for marriage that very day. Nanki-Poo determines he must find Yum-Yum at all costs before the wedding takes place.
Ko-Ko ceremoniously enters and bribes Pooh-Bah into giving him wedding advice. Yum-Yum arrives with her two scatty sisters, Peep-Bo and Pitti-Sing and just as Ko-Ko tries to kiss his unwilling bride-to-be, Yum-Yum bumps into Nanki-Poo. He begs her to turn down Ko-Ko and marry him instead, but she points out that a wandering minstrel is not a suitable husband for a ward of the Lord High Executioner. Nanki-Poo confesses to her that he is, in fact, the son of the Mikado! He had been forced to flee from Titipu when Katisha, an ugly older woman from the Mikado’s court, tricked him into marriage.
Ko-Ko receives a letter from the Mikado stipulating that unless someone is beheaded within a month, Titipu will be reduced to village status. Desperate to avoid this, he searches for a solution which arrives in the person of Nanki-Poo. Devastated that Ko-Ko is going to marry the girl he loves, Nanki-Poo is searching for a tree in order to hang himself but is persuaded instead by Ko-Ko to be his substitute on the chopping block. This is on the condition that Yum-Yum be his bride until his execution one month hence. Ko-Ko grudgingly consents to the marriage, and there is much rejoicing by all.
Suddenly, the evil Katisha appears, claiming Nanki-Poo for herself but is informed that her request can’t be fulfilled because of his upcoming wedding. Katisha mourns her lonely fate before spitefully attempting to reveal Nanki-Poo’s true identity. Yum-Yum, anticipating her intentions, drowns her out in song. A vengeful Katisha is finally pushed offstage by a celebratory ensemble, which insists that nothing should be permitted to ruin the festivities to come.
The women prepare Yum-Yum for the upcoming wedding as she contemplates how much more attractive she is than anyone else in the entire world, but her happiness is shattered when she’s reminded that her husband is to be beheaded in a month. As the wedding procession begins, the company unsuccessfully endeavours to look on the bright side of the situation.
Before the wedding can take place however, Ko-Ko announces his latest discovery: he will not be able to marry Yum-Yum after all, since according to law, when a married man is beheaded, his wife must be buried alive! To Ko-Ko’s dismay, Nanki-Poo decides the best solution for all is not to marry Yum-Yum but to hang himself as he originally planned.
Pooh-Bah announces the approach of none other than the Mikado himself, causing Ko-Ko to insist Nanki-Poo honour his contract and die by the hands of the Lord High Executioner. To his surprise, Nanki-Poo accepts. Ko-Ko admits that in truth, he is incapable of killing even a fly. To convince the Mikado that his orders have been followed, Ko-Ko suggests that they create a certificate of Nanki-Poo’s execution. However, Nanki-Poo must promise to leave and never return. Nanki-Poo agrees to do so only on the condition that he marries Yum-Yum immediately.
The Mikado enters and after demonstrating how to tap any troubles away, is informed by Ko-Ko that the demanded execution has taken place. The Mikado however has come about another matter – his son, who ran away a year ago, has been masquerading in Titipu disguised as a second trumpet player. Ko-Ko, Pooh-Bah and Pitti-Sing try to convince the Mikado that Nanki-Poo is in Cleveland, but Katisha discovers his name on the execution certificate. The Mikado, understanding that Nanki-Poo’s true identity had been kept a secret, regretfully points out that the punishment for killing the Heir Apparent is to be put to death in a cauldron of boiling oil.
Ko-Ko realises that Nanki-Poo must be brought back to life immediately. However, Katisha would then insist on the execution of the newly-wed Nanki-Poo, causing Yum-Yum to be buried alive. Nanki-Poo suggests the ultimate solution – Ko-Ko must marry Katisha! Ko-Ko is reluctant but when Nanki-Poo insists this is the only way he will “reappear” in Titipu, Ko-Ko sets off to win Katisha’s heart.
Katisha wonders why death refuses to come and bring peace to her broken heart as Ko-Ko springs into action, telling her that he’ll die on the spot if she doesn’t accept his love. Katisha claims no one has ever died of a broken heart, so Ko-Ko responds with the tragic tale of Tit-Willow, a little bird who wasted away due to blighted affection. Katisha is touched by the story, and the two realise they are a perfect couple.
As the Mikado prepares to execute the hapless three, Katisha begs him to show them mercy. The Mikado is understanding, but mentions that this does not solve the problem – the Heir Apparent is still slain. With that, the Heir Apparent himself appears, along with the Mikado’s new daughter-in-law, causing everything to fall happily into place as the Mikado calls the show to an end.