Once Upon a Mattress is a musical comedy set in a far away land. Queen Aggravain has taken control of the kingdom, since King Sextimus is unable to speak, as the result of an unhappy curse. In an attempt to keep her son, Prince Dauntless, single, the Queen has decreed that only a true princess who can pass her impossible test may marry her son. Further, until that happens, no one else in the kingdom may marry. The Lords and ladies of the court are in growing worried that no one will ever be able to marry, so brave Sir Harry embarks on a journey and returns with Princess Winifred of the marshlands. Hilarity ensues as a variety of characters try to help the unusual but loveable princess pass the test.