One June day in 1639, at the château of Chaumont in Touraine, young Henri d’Effiat, the marquis of Cinq-Mars, takes leave of his family and sets out, at the request of Cardinal Richelieu, Louis XIII’s chief minister, to join King Louis XIII’s forces at the Siege of Perpignan. Shortly after he leaves, his mother’s guest, Marshal Bassompierre, is placed under arrest at Richelieu’s order and sent in chains toward Paris and the Bastille. Young Cinq-Mars tries to release the marshal, but the haughty old soldier refuses to be rescued. As if his flouting of the king’s officers were not enough for one day, Cinq-Mars returns under cover of night to the château to bid good-bye to Marie de Gonzaga, the beautiful duchess of Mantua, who has been staying with Cinq-Mars’s mother at the château. He returns to bid her farewell because the two, despite the differences of their stations, are very much in love.
Finally leaving Chaumont, Cinq-Mars, accompanied by a few servants, sets out for Loudun. Upon his arrival, he finds the town in turmoil because a local clergyman, a monk named Urbain Grandier, is on trial, accused of being a magician. Charges against the monk have been made by order of Richelieu, who wishes to do away with the independent cleric. The Abbé Quillet, Cinq-Mars’s former tutor, has taken the clergyman’s part and is about to leave Loudun in secret, fearful for his own life. At the execution of Grandier, Cinq-Mars discovers that officials in attendance, the man’s assassins (for they are but that), have given him a red-hot cross to kiss. Cinq-Mars seizes the cross and with it strikes the face of the judge who condemned Grandier, thus earning the enmity of one of Richelieu’s most trusted agents.
After the execution, Cinq-Mars hastens on his way to Perpignan. In the meantime, however, Cardinal Richelieu is making plans to use Cinq-Mars as a tool in undermining the authority of the king. The report of his agents about Cinq-Mars’s actions with regard to the king’s officers and Richelieu’s agents makes no difference to the cardinal, who believes he can shape the young man to his own ends.
Shortly after his arrival at Perpignan, Cinq-Mars is asked to represent the monarchists’ side in a duel against a cardinalist sympathizer. Immediately after the duel, he finds himself in the thick of an attack on the walls of the besieged city, along with the members of the king’s own guard. He behaves so valiantly in the struggle that the captain of the guard introduces Cinq-Mars to the king, much to the disgust of Cardinal Richelieu, who himself had planned to introduce Cinq-Mars to the monarch.
King Louis takes an immediate liking to Cinq-Mars, who has suffered a wound in the battle, and he makes the young man an officer in the royal guards. During the battle, Cinq-Mars had befriended the son of the judge he had struck with the cross at Loudun; the son, a bitter enemy of his father who hated all that his father and Richelieu represented, had approved of Cinq-Mars’s actions. At Perpignan, Cinq-Mars had also renewed a friendship with a young aristocrat named de Thou, who was later to stand as close to him as a brother.
Two years pass, and Cinq-Mars has become the confidant of Louis XIII, has risen to become an important officer in the court, and is now the open and avowed enemy of Richelieu. He hates the minister of state for what he is doing to France; more important, however, is the fact that Cinq-Mars is ambitious to win for himself honors and posts that might allow him to marry Marie de Gonzaga, who, against her will, is being prepared to become the queen of Poland.
To accomplish his ends, Cinq-Mars has earned the king’s confidence and has improved his influence with the nobility and the army. He also has gained the support of the duke de Bouillon, who has been estranged from the king by Richelieu. De Bouillon offers strong support, for he has his own army in southern France. Cinq-Mars also has gained the support of Gaston d’Orléans, the king’s brother and another of Richelieu’s enemies, and of Anne of Austria, the queen, who wishes to protect her children, including the future Louis XIV, from the hatred and ambitions of Richelieu. The success of the plan to depose the minister lies in gaining the king’s support and in securing aid from Spain. Cinq-Mars and his fellow conspirators are forced to deal with Spain on their own initiative, for neither King Louis nor his queen can assume responsibility for bringing Spanish troops into France. In addition, Louis XIII has been under the influence of Cardinal Richelieu and his agents for so long that he has little mind of his own and knows almost nothing of the problems, great and small, that daily beset those who are guiding the kingdom of France in those turbulent years of the 1640’s.
Taking his chances, Cinq-Mars signs a treaty with Spain and sends a copy, concealed in a hollow staff, via a trusted messenger to Spain. Then Cinq-Mars approaches King Louis and secures his royal permission to revolt against Richelieu, after convincing the king that the revolt will not be against the crown. Immediately afterward, as he is leaving the monarch’s apartments, Cinq-Mars realizes that an agent of the cardinal is on his way to seek an audience with King Louis. All Cinq-Mars can do is hope that the king will honor the promise he has given the conspirators.
In order to ensure his union with Marie de Gonzaga, Cinq-Mars has the duchess and himself affianced by a clergyman, an act that is the equivalent of legal marriage. In so doing, however, Cinq-Mars reveals all his plans to the girl in the presence of the priest. Soon afterward, he learns that the priest is not his own agent but is instead a spy for Richelieu. Realizing that his plans are endangered, Cinq-Mars immediately travels to Perpignan, the designated scene of the revolt.
Richelieu has known all the time what is afoot and has made his plans. Having won over the armies, he knows he has nothing to fear in that quarter. He has also arranged for Marie de Gonzaga, in spite of her love for Cinq-Mars, to become queen of Poland. All Richelieu has left to do is to finish off Cinq-Mars and the other conspirators and prevent the treaty from reaching Spain.
The messenger carrying the treaty is intercepted in the Pyrenees by the cardinal’s agents and is killed. In order to gain control of the conspirators, Richelieu pretends to resign his post as minister. King Louis realizes within a few hours that he does not know enough about the affairs of the kingdom to rule France. He calls back Richelieu and grants the minister’s request to do as he pleases with the conspirators. Gaston d’Orléans is banished, while Cinq-Mars and de Thou are arrested at Narbonne, tried at Lyons by a secret court appointed by Richelieu, and beheaded. Marie de Gonzaga, pawn of the cardinal’s political schemes, becomes queen of Poland.