Adapted from the novel by Alexandre Dumas pere, Louis XIII has twin sons, and the one who becomes king is cruel to his people and his brother, who is brought up by the four musketeers and challenges his brother the King.
At Paris in 1638 Fouquet (Joseph Schildkraut) tells Cardinal Richelieu that he would like to be the tutor if King Louis XIII (Albert Dekker) has a son. Louis jokes with D’Artagnan (Warren William) and learns he has a son. The midwife and the doctor call in Colbert (Walter Kingsford) while Fouquet hides and listens as they tell him that the King has twin sons. Colbert tells the Queen that her son belongs to France and takes the second child. He goes out a secret door followed by the doctor and the midwife. Fouquet follows them too.
Colbert tells Louis XIII who is with D’Artagnan that he has twins. Colbert says only one of his sons can rule without conflict between the two. Colbert suggests that D’Artagnan take the child and raise him as his son in Gascony. Louis gives the child to his friend. D’Artagnan leaves with the baby, but first he asks Louis what his name is. Louis names him Philippe.
By the time Louis XIV (Louis Hayward) is 21 years old he is known for his tyranny and cruelty. He bets with Fouquet. Colbert comes in and tells the King that Princess Maria Theresa has arrived in Paris. Colbert tells Louis he should rescind the tax on salt; but the King likes the income, and Fouquet agrees with him. Fouquet warns them about the rebel D’Artagnan who refuses to pay the tax. Colbert says his father exempted him from the tax. Louis goes out. Colbert discusses the secret that Fouquet might betray. Fouquet says the secret made his career, and he will keep it. Fouquet criticizes Colbert’s plan for this marriage to Maria Theresa of Spain. Fouquet tells Colbert that he will hang the King’s twin brother Philippe.
In Gascony at the home of D’Artagnan they drink on the birthday of Philippe (Louis Hayward). The three musketeers, who are the friends of D’Artagnan, are Porthos (Alan Hale), Aramis (Miles Mander), and Athos (Bert Roach). They hear a knock, and the King’s soldiers come in to arrest D’Artagnan. The five engage in a sword fight against the soldiers, but they are outnumbered and are taken to Paris.
Maria Theresa (Joan Bennett) and the Spanish ambassador (Montagu Love) are introduced to Louis XIV, his mother, and Fouquet. Louis says she is beautiful, and he leads her to the dining table. Mlle. de la Valliere (Marion Martin) is introduced. Louis says he is engaged to be married. Maria Theresa is going to be queen of France and is concerned about the state. They hear a crash and learn of a demonstration against the salt tax. Poor people are arrested, and Louis orders the tax doubled.
An old man is being tortured and tells Louis that they are planning to assassinate him in the cathedral. Louis asks the High Constable about it. Louis looks out the window and sees his carriage has arrived. Louis says he must light a candle in the cathedral despite the danger. Fouquet comes in and says he has the Gascony rebels below. Louis signs the order of execution. Colbert says that one of the prisoners looks just like Louis, and he was cheered. Louis tells Fouquet to bring him, and Colbert persuades them to bring them all. Soldiers bring in Philippe and the four musketeers. Louis talks to his twin, and Philippe says he has a scar. Louis orders the others sent to the Bastille. Louis tells Philippe that he usually hangs rebels. The King asks his twin to ride to the cathedral without his mustache and in the King’s clothes.
Philippe has changed and comes out and goes in the carriage. People along the road grumble. A band of men stops the carriage. Philippe fights them with a sword. A man asks the King to kill him, but Philippe refuses to do so. The man tells him about their suffering. Philippe says the King has made many mistakes. The people cheer the King.
In the cathedral Philippe lights the candle and prays. Maria Theresa comes in and prays next to him. She says he proved his worth. They talk intimately, and she gives thanks by lighting a candle.
Louis is talking with Mlle. de la Valliere. Outside the room Maria Theresa tells Fouquet that she wants to see Louis, and he tells her he is busy with Mlle. de la Valliere. She leaves in a huff. She writes a letter that she wants to go back to Spain and sends it with a messenger.
A woman tells Fouquet that Maria Theresa wrote to her father.
Philippe finds Maria Theresa in a garden and tries to woo her. She is surprised that he is being romantic. She wants to hate him but cannot. She says she is a stranger in a strange land. He says kings can make mistakes.
The messenger is stopped on a road.
Fouquet shows the letter to Louis. He complains of her description of him. The Spanish ambassador comes in and complains that the courier was murdered. Fouquet denies it, and Louis apologizes. He asks about the letter and says he will investigate the outrage. The ambassador goes out, and Fouquet and Louis laugh. He rewards Fouquet, who says that she is packing her trunks. Louis says he will pursue an English alliance. Louis says he is going to Fontainebleau.
Maria Theresa is packing, and the Spanish ambassador comes in and tells her that Louis knows the contents of her letter.
Colbert tells Philippe that he knew his father well. Philippe says he never knew him. Colbert says Maria Theresa is going back to Spain. Colbert asks Philippe to bring her back by being King for one day.
Philippe rides fast to overtake the carriage, and they see it is the King. Philippe gets in her carriage and speaks kindly to her. She is angry and does not believe him. He says he never saw her letter. He says the problems of a nation are more important than individuals. They see a young man and woman cavorting outside. Philippe wishes they could escape their destiny and go off together. She would like to believe him. He asks if she dreams while awake. He asks her to dream of a balcony beside him. She believes him.
A priest conducts a betrothal ceremony for Philippe and Maria Theresa, and people outside cheer. Fouquet congratulates Colbert, and they discuss whether it is a comedy or a tragedy.
Fouquet sends a soldier to give a message to Louis about what has happened.
Philippe visits the four musketeers in jail, and they say they are fond of Philippe. He dismisses the guard and shows them his scar. He has come to release them and shows the guard the order. They go out.
Colbert warns Philippe that he must go now, but Philippe says he freed his friends. Philippe must say goodbye to someone first.
Fouquet on horseback tells the captain of soldiers that they must capture the King.
Maria Theresa is writing a love letter to the King and hears a knock. Philippe comes in to says goodnight. She says he looks frightened. He warns her not to trust a king completely and kisses her forehead and hand.
Philippe rides off and is followed by the soldiers. Fouquet shoots at him. Philippe falls into a river. Philippe tells Fouquet to stand aside, but Fouquet persuades him to go with him willingly.
Louis finds Philippe and blames Colbert. Louis wants to hang him and his friends. Fouquet agrees, but Colbert says he cannot shed the royal blood of Philippe and informs them they are twin brothers. Fouquet was also present at their birth. Louis asks for more proof. Colbert goes out and brings in their mother. She embraces Philippe.
Colbert tells Louis not to kill his brother, but he could exile him. Louis gets an idea and laughs.
Louis and Fouquet visit a blacksmith who makes an iron mask. Louis wonders if the beard will cause him to strangle. He will keep the only key on a golden chain.
Philippe in the mask is imprisoned for life in solitary confinement with no visits, but he must be spoken to as a king.
Fouquet tells Louis that Philippe has been in the Bastille for one month. Fouquet wants Colbert to be imprisoned too so that he will not make Philippe king. Louis wants to kill Colbert. Fouquet shows Louis a receipt signed by Colbert. They could add 000 and accuse Colbert of embezzling. Louis says that when Colbert is hanged, he will make Fouquet a duke.
D’Artagnan and his three friends sneak into Colbert’s home. Colbert warns them they must go to save their lives. D’Artagnan asks him if he is willing to let France be controlled by Fouquet. They hear a knock, and D’Artagnan dresses as the servant and answers it. He tells the soldiers that Colbert left for the south of France. They leave. D’Artagnan tells Colbert that they can deliver Philippe. Colbert says they need the key to the mask.
Colbert with them explains to Maria Theresa that she must get the key the King wears around his neck. She agrees.
Louis is drinking with Mlle. de la Valliere, and she leaves him drunk. He hears a knock and lets in Maria Theresa. He is surprised but glad to see her. She is friendly, and they sit down. She hopes to change his mind about the Spanish after she is queen. He kisses her, and she feels the gold chain. He goes into the bedroom and passes out on the bed. She goes in and takes the key and the chain.
She brings the key to Colbert and the four musketeers. Colbert gives it to D’Artagnan and says the destiny of France in his hands. D’Artagnan tells Maria Theresa she is brave.
D’Artagnan and his friends sneak into the Bastille. Inside a guard goes on his rounds. D’Artagnan grabs him and tells Porthos to bind him. D’Artagnan gets the key and opens the cell. He sees Philippe in the mask. He tries to use the key, but Philippe says it is a double lock. The captain of the guard hears a man screaming and goes in and whips him until he is quiet. D’Artagnan says they must get out. D’Artagnan helps Philippe, saying he can be King or be free. Philippe tells him to take him to be King.
Philippe carrying the mask with the other four goes to Louis and wakes him up. Louis pleads he will give them two million francs. Philippe says he left him in despair so he could live in grandeur. He speaks of the law of retribution. Now Louis must suffer for what he has done to the people of France. Philippe offers him a fair exchange. They put the mask on Louis.
D’Artagnan and the others take Louis in the mask to the prison and lock him in. Louis shouts to let him out. A guard tells the captain that men came, but no men are missing. The captain hears the shouting and ignores the ravings of the “king.” He whips him to make him quiet.
Philippe is dressed well and signs a document. Fouquet comes in and asks for his wife to become a duchess. He says that Colbert escaped. He says the tax on salt is reaping double now. Philippe gives him the document that rescinds the salt tax. Philippe says they need salt to eat fish. Fouquet says he cannot do that. Philippe says he could even hang Fouquet. Colbert comes in and bows. Philippe asks Colbert to proclaim the decrees abolishing the taxes. Fouquet asks about the receipt, and Philippe says it was a mistake. Fouquet leaves.
In jail Louis shouts out the window. A plate of food is put under his door. The guard laughs at him because he thinks he is a king. Fouquet throws the food at him. He cleans the plate and engraves a message on it. He hears people outside and throws the plate out his window. A poor man picks it up and reads that he can get a reward by taking it to Fouquet.
Colbert tells Philippe to marry Maria Theresa at once. She comes in and curtsies. Philippe asks her to be queen of France, and she says yes. First she must say if she loves him. She says she always loved him.
Fouquet is getting dressed and is about to leave, and the herdsman with the plate is brought in and asks for money. The servant reads the message that Philippe is sitting on the throne of France. Fouquet sends for his guard and tells them to get the prisoner and bring him to him.
The guards free Louis and take him in a carriage.
In the cathedral Philippe and Maria Theresa walk down the aisle. During the wedding ceremony Fouquet comes in and speaks up. He warns the Cardinal that the man is not the King of France. Fouquet pulls out his pistol and says the King is coming. Mlle. de la Valliere steps forward, and Fouquet kills her and goes out. Philippe says he must go do something.
He and the four musketeers on horses chase Fouquet, who catches the carriage and gets in with Louis who still wears the mask. The five go to the top of a hill and stop the soldiers and the carriage. They fight with swords. Fouquet shoots Porthos. The coach gets by and is chased by D’Artagnan and Philippe. The driver shoots D’Artagnan who throws a knife at the driver, killing him. Fouquet shoots D’Artagnan, who falls down. Philippe helps him, and D’Artagnan says one must lick one’s wounds after the battle. They pursue the carriage again, but it has no driver. The horses become separated from the carriage which falls down the mountain and sinks in the river. D’Artagnan and Philippe see this. D’Artagnan says Philippe has urgent, unfinished business.
In the cathedral D’Artagnan gives the woman, and Philippe and Maria Theresa are married. D’Artagnan collapses and congratulates Philippe. He says his friends are waiting and says, “God save the King.” Maria Theresa says he was a brave man. Philippe says France needs brave women to remind men of their duty.
This classic adventure story contrasts a bad and good king. The beautiful princess naturally hates the bad king and loves the good one. Thanks to the help of the good minister Colbert and the four musketeers, the bad deeds of the bad king and his bad advisor come back to punish them, and the good man will surely be a good king.