In a village in Holland, two American vaudevillians, Con and Kid, who have been travelling in Europe but itch to get back to New York, are stranded penniless in the little inn. As painters and their models sing about the troubles of being a poor artist, the models try to convince the painters to quit their work and have some fun ("By the Side of the Mill"). Overhearing the models complaining, Tina points out that at least the girls have boyfriends. Prompted by this statement Flora discloses that she longs for her painter to say that he loves her, and will one day, marry her ("Loved But Me"). Con and Kid try to sneak out of the inn without paying their bill, but they are discovered and sent to jail. The Innkeeper takes pity on them, however, and arranges for them to work at the inn to pay off their debt.
The Burgomaster's daughter, Gretchen, loves Captain Doris van Damm. Her father, however, wishes her to marry the Governor of Zeeland. Con and Kid agree to help Gretchen and the Captain to elope. Willem tells the Burgomaster about this plot, and he locks Gretchen in the windmill. The Americans try to rescue her, but the Burgomaster has made all arrangements for the marriage between his daughter to the governor. Plotting with Tina, the two Americans finally help Gretchen to escape and then they appear at the wedding festivities (which are missing the bride) disguised as Sherlock Holmes and Watson to "help" Burgomaster find his daughter. Bertha replaces Gretchen as the bride, marries the Governor (her childhood sweetheart), and her identity is then revealed. It turns out, luckily, that Captain Van Damm is heir to a large fortune. The fathers' resistance falls away, and the lovers are united. The Americans return home to New York ("New York").

Question: Why is Gretchen locked in the windmill?
Answer: Because her father learns about the plot to marry her to the Captain and locks her away.
Alternative answer: So she can't elope