An impostor acts as a court jester, is made a knight, and jousts against a challenger for the hand of the princess amid much court intrigue.
During the titles the Jester sings “Life Could Not Better Be.” King Roderick (Cecil Parker) is returning to his castle, but the Black Fox kills one man with an arrow. They hurry to the castle, and the King blames Ravenhurst (Basil Rathbone). They argue about Griswold, and Giacomo (John Carradine) challenges Ravenhurst; but the King stops the sword fight. The King wants Princess Gwendolyn (Angela Lansbury) to marry Griswold, but she refuses. The King blames Griselda (Mildred Natwick). A messenger says he saw the heir with the royal birthmark, the purple pimpernel. The King orders them to catch the Black Fox.
In the forest Hubert Hawkins (Danny Kaye) sings “Outfox the Fox” and is joined by look-alikes dressed in green. The Black Fox (Edward Ashley) arrives and tells Hubert not to wear his clothes. The Black Fox dismisses the midgets and tells Hubert to get the child. Hubert brings the child and shows the purple pimpernel. They learn the King’s men are coming. Hubert tells Maid Jean (Glynis Johns) to take the child to the abbey in Dover the usual way.
Hubert, disguised as an old man, drives a wagon of barrels with “the Captain” (Jean) and the hidden child. The King’s soldiers stop and question them. They are allowed to go on to a woodman’s hut, where Hubert sings a lullaby to the baby. Hubert and Jean lie down next to each other on the straw, and she says she might consider marriage to him. They embrace and kiss, but she says they must win their fight first. She says they need someone in the castle. Giacomo comes in and says he is the King’s new court jester. Jean knocks him out with wood, and Hubert takes his clothes. Jean tells Hubert to whistle in the castle to contact their agent.
In the castle the King complains that the child escaped. The King orders Gwendolyn to marry Griswold. Ravenhurst is counting on Giacomo. Hubert has to repair Giacomo’s wagon, and the King’s men take Jean with beautiful wenches to the castle. The King’s soldiers escort Giacomo’s wagon. Gwendolyn wants Griselda to drink poison with her, but they see the jester arriving. Gwendolyn assumes he is the promised lover of the Princess. Hubert arrives singing “My Heart Knows a Love Song.” He hears whistling and looks at Ravenhurst. They confer, and Hubert asks him to get him into the King’s chambers. The King asks the jester about the Italian court, and the jester makes up an answer. Ravenhurst asks the King to let the jester choose his wenches.
Inside Fergus (Noel Drayton) tries to tell Hubert he is his ally. Griselda comes in and hypnotizes the jester, telling him to make love to the princess. He goes right out. Jean sees Hubert and gives him a key she took from the chamber. The King walks by and orders that Jean sit next to him at the banquet.
Hubert enters the window of Gwendolyn’s room and kisses her hand. He woos her and kisses her. She sees his key and puts it down her dress. He hides before the King comes in. The King says she must marry Griswold. She drops the key, and the King picks it up. The King goes out, and Hubert promises Gwendolyn he will see her later.
Hubert swings on a rope from her window to Ravenhurst’s window. Ravenhurst tells the jester that Brockhurst, Finsdale, and Pertwee must die that night. Hubert swings back to Griselda’s room. She hears the three knights pledge their lives to the marriage of the Princess to Griswold.
Jean tells Fergus to take the child to the jester because he has the key. The King welcomes the jester and Jean, and she sees the King has the key. Gwendolyn tells the jester to meet her at midnight by the north gate. The King asks the jester to sing, and he sings “The Maladjusted Jester.” Griselda drops something into three cups that are served to the three knights. They all toast to the King, and Brockhurst, Finsdale, and Pertwee die. Ravenhurst is impressed and tells the Jester “Plan two.” Jean gives the basket to Fergus for safety. Griswold comes in and says he accepts the King’s terms. The King and Gwendolyn kick the jester. Gwendolyn says she will not marry Griswold because she loves the jester. Griswold is insulted, and the King commands him to stay until tomorrow. He orders the jester put in chains. The King tells Ravenhurst a solution must be negotiated. Ravenhurst learns that the jester is not Giacomo.
Ravenhurst and the captain of the guard suspect that the jester is the Black Fox. Ravenhurst tells the King to knight the jester so that Griswold can challenge him. The winner must marry Gwendolyn. The captain takes Hubert and tests him, saying he passed each time.
Jean goes in to see the King and urges him to dismiss his pages. As she grooms him, she takes the key. She learns the plan to have Griswold kill the jester. The King wants to kiss her, but she implies she has a deadly disease. Jean gives the key to Fergus. She goes to Hubert and warns him he will be killed if he does not flee.
At court soldiers guard Hubert during the ceremony that makes him a knight. Griswold objects and challenges the jester. Jean says the Black Fox will fight in his place, and Hubert accepts. Gwendolyn tells Griselda that if the jester dies, she dies.
Fergus sends a carrier pigeon before Ravenhurst can stop him. In the forest the Black Fox says he will go.
The lists are prepared for the knights. Griselda puts something in a cup, and she tells the jester, “The pellet with the poison is in the vessel with the pestle, not the chalice from the palace.” The jester puts on his armor. Griselda says they changed it, and now “the pellet with the poison is in the flagon with the dragon.” The knights meet, and the King says there will be no toast. The horse rides out from under Hubert. He drops his weapon, and Griswold knocks off his helmet. Hubert pulls Griswold off his horse, and Hubert with his sword spares his life. The captain tells Ravenhurst the child is in the castle.
The King declares Giacomo the winner and says he will wed the Princess, but Ravenhurst accuses him of being the Black Fox and implicates Jean. He says they would put the child on the King’s throne. Hubert sees the midgets in the rafters with ropes. Hubert says he is the Black Fox, and he knocks down Ravenhurst. The midgets attack and toss the soldiers in the moat with a catapult operated by Jean. She has the gate raised, and men in robes enter. Hubert and Ravenhurst duel with swords. Griselda hypnotizes Hubert so he will win. Hubert escapes and embraces Jean, who catapults Ravenhurst into the moat. The Black Fox announces their victory, and Griswold and his soldiers approach. Hubert intervenes and says the child with the purple pimpernel is the true king, and he displays the birth mark.
This musical comedy in a medieval setting entertains as a farce with various plot turns and much silliness, ending up with a baby as the new king.