The Man in the Iron Mask continues the tale of our four heroes from The Three Musketeers, Dumas's wildly popular introduction to the mischievous Musketeers – D'Artagnan, Aramis, Porthos, and Athos. In this dark sequel, we track their lives many years after the prodigious moment when D'Artagnan receives a commission to be a lieutenant in the Musketeers. We find in The Man in the Iron Mask that things have changed quite a bit from the seeming happy days of swashbuckling adventures.
The story opens at the famous French prison known as the Bastille. A priest named Aramis – a former Musketeer – is sitting in a cell with a prisoner. It seems that Aramis is at the prison to hear the man's confession. The prisoner, however, doesn't have anything to confess, because his only crime is being the King of France's twin brother. Aramis happens to be one of the few people in France who knows this secret. Aramis wastes no time in putting together a plan to free this prisoner and swap him for the legitimate king. Once the former prisoner becomes king, Aramis hopes to be rewarded by being appointed adviser to the King, prime minister, or even pope.
Meanwhile, let's get up to speed on the situation with the real King. We have a colorful cast of characters at court. There's King Louis's mother, Anne of Austria, his younger brother (known as Monsieur, with a capital 'M'), his wife Maria Theresa, and his mistress, a woman named La Valliere. Then there's the Superintendent of Finances, a man by the name of Fouquet, who's throwing a party at Vaux in an attempt to ingratiate himself with the King. Among those who would like to see Fouquet swimming with the fishes is a man named Colbert, the Minister of Finances. To round off courtly life, we have D'Artagnan, captain of the King's Musketeers.
Aramis convinces Fouquet to order the release of a prisoner named Seldon. Aramis then arrives at the Bastille, the largest and most feared prison in all of France, and tricks Baisemeaux into releasing the King's brother (his name is Philippe) instead of Seldon. Having successfully freed Philippe, Aramis then convinces him to become king of France. Philippe accepts.
Aramis then briefs Philippe on all the people close to the King so that Philippe can successfully impersonate his twin brother. In the course of this conversation, we learn that a young man named Raoul is dying of a broken heart. He is in love with a woman who goes by the name of La Valliere. Raoul's courtship was going well until the King took an interest in the woman. Being forced to choose between a king and a mere commoner, La Valliere chose the King. Aramis later reveals his ambition to be pope: Philippe can be king of men's bodies, and Aramis can rule their souls. With the diabolical plan agreed upon, the two head for the party at Vaux.
Going to a party at Vaux is a big deal. Everyone who is anyone, is there. Flowers adorn the grounds, the servants outnumber the guests, the food is delicious, the wine flows freely, and the night concludes with a fireworks display. Even though Fouquet and his wife personally wait on the King and Queen, his royal majesty is in a deep sulk. It seems that Fouquet has the money to throw even more lavish parties than the King, and that is not OK.
Before the King goes to sleep, he meets with Colbert, Fouquet's arch enemy. Aramis and Philippe spy on the conversation, and learn that Colbert produced damning evidence that implicates Fouquet in stealing thirteen million Francs from the government. Fouquet seems destined for the Bastille, but the King will make his decision tomorrow.
In the midst of all the partying, the King sneaks off with his mistress and complains that Fouquet is humiliating him. Colbert shows up. La Valliere pleads with the two men to spare Fouquet. Caught up in her love, the King agrees.
As La Valliere leaves, Colbert plants a letter. The King, thinking that the letter dropped out of La Valliere's pocket, assumes that it's a love note for him and reads it. Instead, it's a love letter from Fouquet to La Valliere. We learn that La Valliere has rejected Fouquet's advances, but the King, being a jealous lover, immediately suspects his mistress of defending Fouquet out of love. Fouquet's fate is sealed.
Or is it? That night, the King is replaced by Philippe, the twin brother newly freed from the Bastille. The real King gets thrown into the Bastille, and Philippe morphs into his new royal role. Porthos unwittingly helps Aramis arrange the switch.
The next morning, Aramis goes to Fouquet and tells him to put his mind at rest – everything will be OK. Fouquet naturally wants to know what caused the King's change of heart. Aramis tells him about the change of kings. Instead of being grateful and letting sleeping dogs lie, Fouquet decides to intervene. He gives Aramis and Porthos four hours to get to his estate on Belle-Isle, an impregnable fortress where the two men will be safe from the wrath of the King. Fouquet then heads to the Bastille and frees King Louis. Aramis and Porthos take off as known rebels against the crown.
After an awkward moment when the twins are in the same room, Louis is restored to power. Meanwhile, Porthos and Aramis approach Athos's estate en route to Belle-Isle. They decide to pay their old friend a visit. Athos has been consoling his son, Raoul, whose heart was broken by La Valliere. Porthos still believes that he's accompanying Aramis on a secret mission for the King, a mission for which he will be handsomely rewarded. Aramis tells Athos the real reason behind their flight from Paris. Aramis and Porthos take horses from Athos and continue their journey to Belle-Isle.
Monsieur le Duc de Beaufort is the next visitor to Athos's home. He is about to embark on a military campaign to Africa and wants to say good-bye. Raoul immediately begs permission to serve as the Duke's aide-de-camp. Athos is deeply upset because it's clear to him that Raoul is going into this war with a death wish, but agrees to let Raoul go.
Beaufort directs Raoul to the coast of France to prepare the armed forces. Athos accompanies his son to the coast. Before they leave, they search out D'Artagnan to say their good-byes. Unfortunately, D'Artagnan has already left Paris. After a little sleuthing, Athos and Raoul determine that D'Artagnan is heading to Cannes. Luckily, Cannes is along their route. The two men assume they will meet D'Artagnan on the way.
As they head to the coast, Athos and Porthos don't hear anything about D'Artagnan. Athos assumes D'Artagnan is being deliberately secretive while on a mission for the King. A chance encounter with fishermen convinces Athos that D'Artagnan is on the island of Ste. Marguerite, forcing Philippe into a life of captivity. The two men go to the island and catch up with D'Artagnan. They also meet Philippe, who is now forced to wear an iron mask and is forbidden from communicating with anyone. During the visit, D'Artagnan is summoned back to the Louvre to serve the King. The three leave the island together. As D'Artagnan makes his way to Paris, he prays his next mission won't involve bringing in Porthos and Aramis. Meanwhile, Athos is forced to say a painful good-bye to his son.
At the Louvre, D'Artagnan meets La Valliere, and shames the woman by talking about Raoul's determination to die in battle. D'Artagnan meets with the King and learns of His Majesty's intention to hold meet with a group of nobles at Nantes.
Meanwhile, Fouquet, nervous that he'll be arrested any minute, has worked himself into a grave illness. Friends attempt to console him by concocting escape plans. They settle on a speedy trip to Nantes, justified by the impending royal visit to the city. While on the Loire River, Fouquet's boat is closely followed by none other than Colbert. Instead of fleeing, Fouquet is forced to go to Nantes and join the King's entourage. Upon his arrival, D'Artagnan shows up to warn him that once the King arrives, Fouquet will unable to flee.
Unfortunately, D'Artagnan's warning comes too late. The King arrives and all hope of Fouquet's escape is lost. The King gives D'Artagnan explicit orders to arrest Fouquet and, despite D'Artagnan's inclinations to the contrary, he obeys. A massive chase scene ensues. D'Artagnan is completely wiped out by the end of it, and although Fouquet could have made his escape, the two men go back to the Louvre. The King then orders D'Artagnan to capture Belle-Isle. D'Artagnan is unhappy to hear this order – Aramis and Porthos are likely to defend Belle-Isle to the death.
On Belle-Isle, Aramis is obliged to tell Porthos the whole truth behind their flight. D'Artagnan comes over for a visit and three friends attempt to formulate a way out of their situation. After conferring with his friends, D'Artagnan heads back to the small army he currently commands. Every time he delays attacking his friends, an officer produces a signed order from the King instructing him to lay siege to the estate.
As a last resort, D'Artagnan resigns, assuming that his army must accompany him back to Nantes, leaving Aramis and Porthos free to escape. Unfortunately, the King also anticipated this move: an officer arrests D'Artagnan and someone else assumes control of the army. As D'Artagnan is escorted back to France, he can hear the cannons firing on Belle-Isle.
As the fighting breaks out, Porthos receives a premonition that he is going to die. He is correct. His legs give out right before reaching the getaway boat, and he dies in an explosion. Before his death, however, Porthos took out a hundred and ten of the King's men. Aramis was also crucial in the battle, he was the brains behind the whole fight.
D'Artagnan is angry when he reaches Nantes. He becomes even angrier when the King refuses to see him. D'Artagnan resigns so he can be considered a private citizen. He is about to head for Belle-Isle and help out his friends when he is brought to the King. The two men face each other down and the King manages to sway D'Artagnan to his side. Upon D'Artagnan's request, the King pardons Aramis and Porthos.
D'Artagnan takes off for Belle-Isle to inform his friends. He fails to learn their whereabouts, but receives a letter from Aramis, currently hiding in Spain, bringing him up to speed. When D'Artagnan returns to Nantes, he learns that the King had intercepted that letter. D'Artagnan also learns that Colbert, had interceded on Aramis's behalf.
Meanwhile, Fouquet's weeping friends seek an audience with the King. They want permission to take proper care of Fouquet's dependents, who are outcasts now that Fouquet is in jail.
Porthos's funeral is held at his estate in Pierrefonds. His will is read. He leaves everything to Raoul, with D'Artagnan, Aramis, and Mouston also mentioned. After the funeral, Mouston, Porthos's life-long servant, dies of grief.
Back in Blois, Athos is dying of a broken heart. He cannot bear being separated from his son. He dies as soon as he learns that Raoul has been killed. Minutes later, D'Artagnan walks into the room. Athos and Raoul are buried side by side on the edge of Athos's estate. D'Artagnan finds La Valliere crying over the graves, and shames her some more, telling her that she is responsible for the deaths of two fine men. La Valliere begs forgiveness and assures D'Artagnan that she too will suffer.
Four years later, D'Artagnan has been made into a count. The King has moved on to a new mistress, Aramis returns to France as a Spanish duke and ambassador, and Colbert has been busy with various government projects. France is going to war with Holland. D'Artagnan heads off to war in the hopes of becoming a marshal of France. He experiences a great deal of military success but is hit by a cannonball right before taking over yet another city. Out of the four original Musketeers, only Aramis remains alive at the end of The Man in the Iron Mask.