The Three Musketeers

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In 1625 France, following in his late father's footsteps, d'Artagnan (Chris O'Donnell) sets off to Paris in hopes of becoming a member of the Musketeers, a band of men sworn to serve and protect the King of France. d'Artagnan is pursued by GÃ©rard and his brothers, who accuse him of blemishing their sister's honor. GÃ©rard saw his sister kissing d'Artagnan goodbye or as d'Artagnan put it "she wanted to give me something to remember her by!" At Musketeer Headquarters, Captain Rochefort (Michael Wincott) and the Cardinal's Guards have disbanded the Musketeers as per the orders of Cardinal Richelieu (Tim Curry), the King's Minister, ostensibly to help fight in an impending war with England. Rochefort confides to the Cardinal that there are three Musketeers that have refused to relinquish their duties: Athos (Kiefer Sutherland), Porthos (Oliver Platt), and Aramis (Charlie Sheen).
Upon reaching Paris, the headstrong d'Artagnan has a series of chance physical encounters with these same three Musketeers, resulting in d'Artagnan's accepting a duel with each one that very day. D'Artagnan arrives at the Ruins for his first duel; and, much to his surprise, Athos, Porthos and Aramis reveal themselves as Musketeers. But, before the duels can commence, the Captain of the Cardinal's Guard appears with orders to arrest the resistant Musketeers. Although d'Artagnan himself is not under arrest, he joins the three Musketeers in the ensuing skirmish, aligning himself with them. Displeased (but still impressed) by d'Artagnan's involvement, the Three Musketeers leave d'Artagnan behind. More of the Cardinal's Guards, led by Rochefort, arrive; and d'Artagnan is captured.
During an escape attempt, d'Artagnan is able to eavesdrop on a conversation between Cardinal Richelieu and Milady de Winter (Rebecca De Mornay), as the Cardinal asks that she deliver a signed treaty to the Duke of Buckingham of England. Before he can get a view of the Cardinal's spy, d'Artagnan is caught at the doorway by Rochefort, interrogated by the Cardinal, and ultimately sent for execution the next morning. At the execution, d'Artagnan is saved by Porthos and Aramis; and the three make a getaway in the Cardinal's personal coach, driven by Athos. While d'Artagnan reveals Richelieu's plans, the Three Musketeers decide to intercept Richelieu's spy to prove that the Cardinal is guilty of treason.
That night, d'Artagnan and the three musketeers stop at an inn to rest. Athos tells the story of a Count who fell in love with a beautiful woman; but, upon discovering that she was branded for execution, he betrayed her by giving her up to the authorities. The party decides to split up during a skirmish. Athos sends d'Artagnan to ride ahead and intercept the Cardinal's spy and the treaty, but d'Artagnan passes out from exhaustion in the middle of the road. When he wakes up, he finds he is stripped of his weapons and clothes and Milady de Winter is there to rouse and seduce him. Not knowing who the spy is, d'Artagnan tells her of his plans, whereupon she tries to kill him. Instead, d'Artagnan convinces her to keep him alive. When Milady de Winter's party tries to escape by boat to England, they find that the crew has been killed by Porthos and Aramis, and another skirmish ensues. Milady attempts to run away; but she is confronted by the newly arrived Athos, who recognizes her and calls her Sabine. He is astonished to see her, as he thought she was dead. It is revealed that he was the Count of his story and that Sabine was the wife he betrayed. Milady de Winter is then apprehended by her former brother-in-law, as Sabine is responsible for killing her husband, Lord de Winter, and sentenced to death by execution.
The three musketeers retrieve the treaty and learn that the Cardinal is planning something on King Louis' birthday, though it does not specify what in the treaty. Athos attempts to learn what it is by visiting Sabine in her cell. She asks if he can stop her execution tomorrow. Athos cannot and Sabine does not reveal what the Cardinal's plan is. During the execution, just as Sabine is lowering her head for the executioner, Athos stops him and begs forgiveness from Sabine for his betrayal. She accepts and whispers to Athos Richelieu's plans to assassinate King Louis before jumping off a cliff to her death. After learning of the Cardinal's plan, the three musketeers set out to re-band the rest of the musketeers, in secret, for the king's birthday celebration. Richelieu and Rochefort hire a sharpshooter to assassinate the king. During the assembly, D'Artagnan is able to stop the sniper from killing the king, but the shot narrowly misses its target and the Cardinal blames the musketeers in the crowd for the attempted assassination.
Athos, Porthos and Aramis drop their cloaks to show their musketeer tunics and face the Cardinal's guards. Meanwhile, men from the crowd rush to their sides and reveal that they are musketeers. A battle between the musketeers and the Cardinal's guards engulfs the palace. Richelieu takes the king and queen as hostages and tries to take them to the dungeon below. Aramis confronts the Cardinal to stop him, but Richelieu shoots him in the chest with a pistol and makes his way into the passage to the dungeon. Athos duels Rochefort and D'Artagnan interrupts the battle to fight Rochefort himself. During D'Artagnan's duel, Rochefort reveals that he was the one that murdered D'Artagnan's father, and D'Artagnan, from anger, renews his efforts to kill him. Rochefort fights back and is able to disarm D'Artagnan. Just as Rochefort is about to deal D'Artagnan the final blow, D'Artagnan's sword is jettisoned back to him and Rochefort is killed before he can strike. D'Artagnan has finally avenged his father's death. Constance, on the stairs, slides the sword away and puts her hands in his, smiling.
Athos joins Porthos, who is at the unconscious Aramis' side, and as they search for his wound, Aramis suddenly wakes, and it is revealed that the bullet was stopped by the huge cross that Aramis wears. They follow Richelieu into the dungeons and split up to stop him from killing the king and queen. In the dungeon, Porthos is confronted by the brutal jailer but after a brief fight, manages to defeat him. Afterwards, Athos and Porthos just miss the Cardinal as his boat starts on the underground river. Athos says that they have proof of Richelieu's treason, but Richelieu does not seem to care. The boatman then casts off his cloak and the Cardinal is astonished to see that it is Aramis. Aramis attempts to apprehend the Cardinal, but King Louis stops him and punches Richelieu, knocking him in to the river. It is the last time Richelieu appears in the film.
The musketeers are reinstated by the king. Accompanied by Athos, Aramis, and Porthos, D'Artagnan is honored in a ceremony. King Louis offers him anything he wants, and all D'Artagnan wants - at Athos' request - is to serve Louis as a musketeer. King Louis does so. Constance, who has remained by the queen's side, runs to him and gives him a passionate kiss, impressing both Aramis and Porthos. Outside Musketeer Headquarters, GÃ©rard and his brothers once again challenge D'Artagnan to an immediate duel. D'Artagnan tells his new friends that he will take care of this problem and Porthos stops him from continuing, stating that in addition to protecting King and country, musketeers protect each other. D'Artagnan calls out, "All for one..." and the rest of the musketeers shout out, "One for all!" The scene ends with GÃ©rard and his brothers being chased by the entire division of musketeers.

Question: What do the Three Musketeers decide to try to prove the Cardinal guilty of?
Answer: Treason