Made for a cost of $4 million in the fall of 1955, it was the most expensive comedy film produced at the time. The motion picture bombed at the box-office on its release, bringing in only $2.2 million in receipts the following winter and spring of 1956. Since then, it has become a television matinee favorite. The film contains the famous exchange: "The pellet with the poison's in the vessel with the pestle; the chalice from the palace has the brew that is true!" (mainly between Kaye and Mildred Natwick as Griselda).
When the Black Fox sends a warning to the tyrannical King Roderick (Cecil Parker), the usurper who was responsible for the massacre of the true king and his family, he grows seriously fearful of his unsteady position, demanding answers from his chief advisers, Lords Brockhurst (Alan Napier), Finsdale (Lewis Martin), Pertwee (Patrick Aherne), and the evil Lord Ravenhurst (Basil Rathbone), who is the real power behind the throne. All, except Ravenhurst, suggest an alliance with the northern "barbarian" Sir Griswold of Mackalwane (Robert Middleton), who has enough troops in his employ to deal with the situation. All they would need in exchange is giving him Princess Gwendoline's (Angela Lansbury) hand in marriage. Princess Gwendoline however, refuses to marry the "Grim and Grisly, Gruesome Griswold", telling the king her nanny, the witch Griselda (Mildred Natwick), has told her stories of romantic love and promised such a future for her. When the king orders Griselda's execution, Gwendoline threatens suicide. Upon then, hearing of a survivor amongst the true king's family, King Roderick demands the death of the Black Fox.
In the introductory scene — a musical song and dance — the audience is led to believe that Kaye is playing the part of the Black Fox, with his assistants, a band of dwarfs (Hawkins' pals from the carnival), all in identical costumes. It is only after the end of this production that the real Black Fox appears, and suggests that Hawkins should stay out of his wardrobe, having been previously warned. Kaye responds that he is only trying to improve the morale of the troops. Hawkins tells the Black Fox of the usefulness of his dwarf friends for the cause, but the Black Fox kindly dismisses them, not taking them seriously. The dwarves leave, remembering the Black Fox to summon them if the need arises.
Hawkins is given a serious task to perform: when the location of their camp becomes known to the king, he must escort the rightful heir to the throne, a baby that bears the royal birthmark on his posterior — a purple pimpernel — and Maid Jean (Glynis Johns), the Black Fox's right-hand woman and the object of Hawkins' affections, to safety. To achieve this, he disguises himself as an asthmatic old man, the grandfather of the peasant maiden played by Jean. After safely eluding the king's forces, Hawkins and Jean are forced to stay in a hut during a thunderstorm, where Jean tells him of the existence of a secret underground tunnel leading from the forest to the castle. If it were opened, the Black Fox's forces could launch a surprise attack on the usurping King, but there is a problem: it only opens with a key that the king has in his possession.
Unexpectedly, a golden opportunity comes to them: a stranger (John Carradine) enters the hut to seek shelter. He introduces himself as the king's new talented and multilingual jester, Giacomo, "King of Jesters, and Jester of Kings". Giacomo is scheduled to entertain King Roderick at the upcoming tournament. Hawkins and Jean realize the potential; Jean (Hawkins proves to be reluctant) knocks the jester out, and Hawkins assumes his identity. Jean tells Hawkins to meet the Black Fox's spy at the castle and listen to him. As Hawkins, in the guise of Giacomo, fixes a wheel, Jean attempts to transport the royal heir to the abbey in Dover, a safe haven. However, she and the hidden infant are picked up by the king's soldiers, who are roaming the countryside for fair maidens as playgirls for the tournament, and taken to the castle themselves. Upon arriving, Jean uses the secret call (a whistle) to contact the spy, Fergus the hostler (Noel Drayton). Jean tells Fergus about "Giacomo" and tells him of the plan.
Lord Ravenhurst's advisors are concerned with his "compliance" with the king's other advisers. Ravenhurst, however, is confident he will be the ultimate adviser to the king, as he has planned for an assassin-Giacomo-to kill the other advisers and abduct Princess Gwendoline. Meanwhile, the witch Griselda is summoned by Princess Gwendoline, who is angry about the arranged marriage to Griswold. The Princess threatens to kill Griselda for lying to her about her destiny. Griselda, failing to hypnotize the Princess with her evil eye, and desperate, witnesses Hawkins' arrival to the castle and proclaims him to be Gwendoline's romantic suitor. Princess Gwendoline immediately takes a fancy to the new jester, telling Griselda that if she is lying the princess will kill her herself.
Hawkins, infiltrating the evil King Roderick's castle, searches for the spy, incorporating the secret whistle into his opening song. Just as Fergus answers him, Hawkins turns, but instead sees Ravenhurst. Believing Ravenhurst to be the spy, he complies with his orders. Meanwhile, with his knack for comedy, he manages to worm his way into the King's confidence.
In "Giacomo's" chambers, Fergus tries to tell Hawkins that he is the true spy, but before he can explain Griselda arrives, asking "Giacomo" to meet the princess. When Hawkins politely declines, she uses her "evil eye" to put him under a spell instead. Once hypnotized, Hawkins can be brought in and out of a trance at will by a snap of the fingers, and Griselda sends him to court the Princess. Jean gets the key which opens the tunnel door and paves the way for a daring invasion of the castle. Meeting Hawkins, unaware that he is hypnotized, gives him the key. King Roderick then arrives and, seeing Jean, deems her perfect for his escort at the night's festivities. Jean is then rushed off to be cleaned and dressed.
Hawkins, under his trance, meets with Princess Gwendoline and seduces her. Assuring her of his love, he gives her the key. Gwendoline is now completely entranced with "Giacomo" and plans their escape. However, King Roderick arrives and Gwendoline hides Hawkins. She pretends to comply with her father's wishes, but the king finds her hiding the key. The secrecy is almost threatened when she and the king get involved in a snapping match, but the king misses Hawkins. While still under the trance, Hawkins also visits Lord Ravenhurst and his henchmen, assuring them of their plan's success. Returning to his quarters, Griselda puts him into a deep sleep, remembering nothing. As she leaves, she overhears Lords Brockhurst, Dinsdale, and Pertwee vow that they will uphold the marriage of Princess Gwendoline to Sir Griswold no matter what.
Hawkins awakens in costume to hear everyone tell him about the "secret plans", although he is absolutely clueless. Before he knows it, he is handed the royal heir (in a basket) by Fergus, and thrust into the main dining hall in front of everyone. On the spot, he performs an impromptu act that has the royal court rolling, while able to signal to Jean about the basket, enabling her to take the basket back to Fergus.
Hawkins proposes a toast to the king. While he does this, Griselda poisons the vessels of Brockhurst, Dinsdale and Pertwee. When they die, the delighted Ravenhurst accredits "Giacomo" to the deed. The Princess's suitor, Sir Griswold of Mackalwane arrives to receive his gift in exchange of the alliance: Princess Gwendoline. Gwendoline then announces that there will be no marriage, for she is in love with "Giacomo". The King, enraged, orders the jester's death. Gwendoline threatens suicide. Griswold, insulted, announces that, if "Giacomo" were an "equal" knight, he would challenge him to mortal combat. Meanwhile, Ravenhurst and his fellows have learned that Hawkins is an impostor.
Hawkins is first thrown in jail. In the meantime, Ravenhurst and his fellows, along with the Captain of the Guard (Herbert Rudley), suspect that he is the Black Fox himself. They decide to exploit this to help their own plans, and upon Ravenhurst's suggestion, tell the king that the answer to the problem is to knight Hawkins so that he can be legitimately challenged to a joust by Sir Griswold and killed.
Hawkins is swiftly put through the initiation of knighthood. The challenges of knighthood are made easy for him (he is given no chance to not succeed). He is warned of the plot by Jean, who has managed to get the key again, but while trying to escape, he finds himself walking into the ranks of knights marching to his own investiture. A slow, stately, time-honored ritual, the knighting ritual is accelerated to breakneck speed as the king fears inclement weather outside.
Immediately upon being knighted, Hawkins is challenged to a duel to the death by Griswold. Jean informs Fergus of the news, who sends a note, along with the key, to the Black Fox. After doing so, Fergus is captured by Ravenhurst and his fellows.
Help is given to Hawkins in the form of a poisonous potion made by Griselda, but Sir Griswold is also told of it by a courtier who overhears of the plot.
The switch with the poisoned drink builds to the following and famous dialogue exchange:
In a scene full of tongue twisting English, both combatants approach the King, each trying desperately to remember which cup contains the poison, with the result that the king decides to abandon the ritual of drinking a toast before the joust and get straight on to the fight. But, as a storm is rising, Hawkins' armor has being previously struck by lightning and became magnetized. Griswold is pulled from his horse by his morning star being magnetized, which sticks to Hawkins' shield, and Hawkins wins the joust. He passes on delivering the coup de grâce, sparing Griswold.
In the forest, the Black Fox and his men prepare to sneak inside the castle, but then the secret passage collapses, leaving only an opening "barely enough for a child". The Black Fox immediately summons Hawkins' dwarf friends to attack the castle first and open the drawbridge so that the rebels, disguised as monks, can assault from the countryside.
Ravenhurst and his fellows have tortured and killed Fergus, and have learned that the royal heir is also inside the castle. As Hawkins is declared victor, he is immediately unmasked as the Black Fox by Ravenhurst and arrested. As he and Jean stand trial, however, they are freed by Hawkins' friends, the dwarves, disguised as the king's men. Hawkins confesses to being the Black Fox, and the dwarves rescue the heir, simultaneously opening the gate to the castle for the Black Fox with the help of Jean.
The rebels attack the castle. When Hawkins must defeat the villainous Ravenhurst at swordplay, he is again entranced by Griselda into thinking that he is a great swordsman. He duels with Ravenhurst while alternating between two personalities: the true Hawkins, who is an extremely coward and a clumsy fighter, and the entranced version, who has the skill and demeanour of a master swordsman. The spell can be reversed and re-implemented every time someone snaps their fingers; thus, since both Hawkins and Ravenhurst snap their fingers to illustrate every point they make, Hawkins changes between the two very frequently. Eventually, however, with the help of Jean, a catapult and several of his dwarf friends, Ravenhurst is defeated.
As the usurpers of the throne are defeated by the Black Fox and his army of dwarfs, Griswold returns to defend the king's honor. However, Hawkins arrives and shows Griswold the rightful King, revealing him by his distinctive birthmark and acknowledged by good and bad alike. Hawkins embraces Jean and leads the entire cast in one last chorus of life couldn't possibly better be.
Note: Sylvia Fine was Danny Kaye's wife.