Cary Grant and Sylvia Sidney star in a screwball comedy about love and mistaken identity in Thirty Day Princess. When Princess Catterina (Sylvia Sidney) of Taronia falls ill with the mumps, it threatens to derail a business deal worth $50 Million. Afraid of losing his commission, financier Richard Gresham (Edward Arnold) hires look-alike actress Nancy Lane (Sidney again) to impersonate the princess and promises a bonus if she can get crusading newspaper publisher Porter Madison (Cary Grant) to drop his opposition. Just as love begins to bloom between the princess and the publisher, Catterina's fiance arrives unannounced threatening to ruin everything.