Despite his recent ascension to the throne via his massacre of the royal family, the crown of the tyrannical King Roderick of England weighs heavy, as rumors persist throughout his kingdom that a true heir to his title lives, a male infant who bears the royal birthmark, that of a purple pimpernel. Roderick's fears are well-founded, as one of his spies informs the king that such a child does exist and is being cared for in the hidden forest lair of the masked thief known as The Black Fox. Despite the objections of his top advisor, Sir Ravenhurst, Roderick hopes to secure his crown by making a marriage alliance between himself and a powerful Northern knight, Sir Griswold. Princess Gwendolyn, the king's daughter, rejects such an arranged marriage, proclaiming that she will only marry for love, as prophesized by the witch Griselda. Meanwhile, back in his forest hideout, The Black Fox is warned that the king's soldiers are nearby, so he orders Hubert Hawkins, an ex- carnival performer, and the maid Jean, a captain in the Fox's army, to transport the child king to an abbey in Dover. Disguised as a deaf old man and his mute granddaughter, Hawkins and Jean manage to safely elude the king's men. Later, they are forced to seek shelter from a sudden rainstorm in a woodsman's hut, where Hawkins and Jean proclaim their love for each other. Their romantic interlude is interrupted, however, by the arrival of Giacomo, "King of Jesters and Jester of Kings." After subduing the jester, Hawkins assumes Giacomo's identity in order to gain access to Roderick's court, unaware that he is impersonating a deadly assassin hired by Ravenhurst to kill his rivals. Matters are further complicated when Hawkins mistakes the evil Ravenhurst for an agent of the Fox, and Griselda, fearful for her life if her mistress Gwendolyn is forced to marry Griswold, hypnotizes Hawkins into believing he is a dashing swashbuckler and orders him to woo the love-sick princess. Meanwhile, Jean and the infant king, who is hidden in a wine casket, are captured by Roderick's men, who have been ordered to search the countryside for beautiful wenches meant to provide "entertainment" at a royal banquet in Griswold's honor. That night, Griselda poisons Sir Brockhurst, Sir Finsdale and Sir Pertwee after the three pledge their lives to the consummation of Gwendolyn and Griswold's marriage, though Ravenhurst mistakenly credits Hawkins with their deaths. Soon thereafter, Gwendolyn publicly proclaims her love for the jester, so the insulted Griswold challenges Hawkins to a duel. In order to meet the rules of chivalry, the commoner Hawkins is then rushed through the sacramental rights of knighthood. Meanwhile, Jean procures from the lecherous Roderick the key to a secret passageway into the castle, and sends it by messenger pigeon to the Black Fox, requesting that he fight in Hawkins' place. A cave-in makes the hidden tunnel seemingly impassable, however, so Hawkins is forced to face Griswold in mortal combat. Though Griselda's attempt to poison the Northern knight fails, Hawkins manages to best Griswold after his armor is magnetized by lightning. Before the victorious Hawkins can claim Gwendolyn as his prize, however, Ravenhurst accuses him of being the Black Fox. Tried before the royal court, Hawkins and Jean are saved from execution when the real Black Fox infiltrates the castle with the help of Hawkins' midget friends. During the ensuing battle, Hawkins slips in and out of Griselda's spell long enough to defeat Ravenhurt in a sword fight. With Roderick's forces now vanquished, the infant king is placed on the thrown of England, and Sir Hawkins' is rewarded with the hand of Jean.