Cristina, the princess of a fictional monarchical state named Saint Tulipe, is visiting Rome with her father, Gustavo VI, who is trying to appeal to Italy's national bank for a loan to buoy his country's financial crisis.

Barnaba, a charming and happy-to-go ATAC employee, takes Cristina around the city, including the Roman Forum, and slowly begins to win her personal interest.

However, after having had her fun and spending a (chaste) night in his apartment, she locks him into his clothes dresser's massive drawer and leaves with her family's bodyguards without telling him about her true identity.

When Gustavo refuses, Barnaba engages in a series of daredevil schemes to prove his worth, even to the point of appearing (by Cristina's invitation) at a royal banquet in his ATAC uniform and holding successful discussions about high politics with a number of international dignitaries.

However, although Barnaba succeeds in impressing Gustavo and winning Cristina's genuine affection, Saint Tulipe's financial crisis finally drives the king into engaging his daughter to an arms manufacturer.