Whilst Ko-Ko is lamenting the position as "simply appalling" he is disturbed by the entrance of Nanki-Poo with a rope in his hands. He has made up his mind to commit suicide because Ko-Ko is going to marry Yum-Yum. Finding "threats, entreaties, prayers all useless" Ko-Ko is struck with a brilliant idea. He suggests that Nanki-Poo should at the end of a month's time "be beheaded handsomely at the hands of the Public Executioner." To this Nanki-Poo agrees on condition that Ko-Ko permits him to marry Yum-Yum. Reluctantly Ko-Ko accepts the condition, and when Pooh-Bah returns to enquire what Ko-Ko has decided to do in regard to an execution, he replies, "Congratulate me! I've found a volunteer." Whilst the townsfolk of Titipu are bantering Nanki-Poo on the prospect of marriage and death, their revelry is interrupted by the arrival of the lady who was the cause of Nanki-Poo's wandering. Katisha discovers Nanki-Poo and calls upon him to "give me my place." When he refuses she would have revealed his identity, but every time she tries to say "He is the son of your Mikado" her voice is drowned by the singing of Nanki-Poo, Yum-Yum and the chorus. Eventually Katisha rushes away threatening furious vengeance.
When the curtain rises again the scene is the garden of Ko-Ko's palace. We see Yum-Yum decked by her bridesmaids for the wedding, while they sing of her loveliness, and Pitti-Sing bids her "Sit with downcast eye; let it brim with dew." Pitti-Sing tells her also that "modesty at marriage tide well becomes a pretty bride," but this admonition seems lost upon a bride who, when her adornment is complete, frankly revels in her beauty. In "The Sun whose rays," a song of entrancing melody, she declares, "I mean to rule the earth as he the sky."
But her rapture is marred by the reminder from Peep-Bo that her bridegroom has only a month to live. Nanki-Poo finds her in tears, and has much difficulty in comforting her, their feelings being aptly expressed in that wonderful madrigal, which although it begins so joyfully with "Brightly dawns our wedding day," yet ends in tears. Ko-Ko now joins the wedding party, and although the sight of Yum-Yum in Nanki-Poo's arms is "simple torture," he insists on remaining so that he may get used to it. When Yum-Yum says it is only for a month, he tells of his discovery that when a married man is beheaded his wife must be buried alive. Naturally, Yum-Yum demurs to a wedding with such a hideous ending to the honeymoon, and Nanki-Poo declares that, as he cannot live without Yum-Yum, he intends to perform the "happy dispatch." Ko-Ko's protest is followed by the entry of Pooh-Bah to announce the approach of the Mikado and his suite. They will arrive in ten minutes. Ko-Ko, believing that the Mikado is coming to see whether his orders regarding an execution have been obeyed, is in great alarm. Nanki-Poo invites Ko-Ko to behead him at once, and Pooh-Bah agitatedly urges Ko-Ko to "chop it off," but he declares that he can't do it. He has "never even killed a blue-bottle." Ko-Ko decides that the making of an affidavit that Nanki-Poo has been executed, witnessed by Pooh-Bah in each of his capacities as Lord Chief Justice, etc., etc., will satisfy the Mikado. Pooh-Bah agrees on condition that he shall be "grossly insulted" with "cash down."