Once Upon A Mattress is based on the famous fairy tale, The Princess and the Pea. Because of her noble birth, the Princess is unable to sleep on a bed of many mattresses when a tiny pea is placed underneath the bottom mattress.

The kingdom is an unhappy one. King Sextimus has been struck dumb by a witch's curse and is condemned not to speak again until "the mouse devours the hawk!" Queen Agravia has assumed power and she talks enough for the whole Royal household. She has decreed that no one in the Kingdom may wed until Prince Dauntless is married to a true princess of Royal blood. So far the Queen has tested the eligibility of eleven applicants and each has failed the almost impossible tests the Queen has devised.

The Knights and Ladies of the Court are most distressed by the situation but the most concerned are Lady Larken and her handsome lover, Sir Harry. Lady Larken has informed Sir Harry that he is to be a father so the situation is quite desperate. Sir Harry, determined to find a true princess, sets off for, until now, unexplored parts and returns with Princess Winnifred. She arrives dripping wet have swum the moat and proceeds to act as unlike a princess as one could possibly imagine. Prince Dauntless is very impressed by the new princess and is soon obviously in love with her - to the consternation of the angry Queen, who decides to give her an impossible test. She consults the Wizard and together they decide on the the sensitivity test - a pea is placed beneath twenty mattresses and the Queen decrees that if Princess Winnifred cannot sleep she will prove her royal birth. However, to ensure that she does sleep, she proceeds to do everything to make her tired out. A ball is held where the dancing is fast and furious and Winnifred is encouraged to drink heavily. She is also given a sleeping draught and a singing bird is provided to sing her to sleep.

However, the Queen fails, for Winnifred cannot sleep and spends the night counting sheep. The Queen is furious and orders Princess Winnifred to leave, but Prince Dauntless defies her and puts an end to the curse upon the King for, as the Jester proclaims, "the mouse has devoured the hawk!"

At the final curtain, we discover that the Princess was unable to sleep because the Jest has placed under the top mattress his lute, a helmet, a large spiked ball, some livestock and some old armour. Of course, all ends happily and the Queen is punished.