Explorer Emile Bonnaire arrives at the port town of Le Havre from the New World and goes to a tavern. D'Artagnan is watching for him and notices three thugs that are also after the explorer. The Musketeer follows them all inside where Bonnaire orders a round of drinks for the bar patrons. Porthos, Athos, and Aramis are inside and D'Artagnan joins Athos at a table. They notice two other black-clad men, Spanish spies, enter the tavern as well.
As Bonnaire flirts with a barmaid, a woman named Marie comes in and threatens the explorer's life. She grabs the barmaid and prepares to stab her with a knife, while the three thugs move in on Bonnaire. Bonnaire shoots one of them with a hidden gun while the Musketeers dispatch of the other two. They run out while Aramis tries to restrain Maria and gets a slap for his efforts. Bonnaire introduces Maria as his wife and Athos informs him that he's under arrest. They have orders to take him to King Louis and won't take no for an answer. The two spies step forward and one of them offers Bonnaire his map case back, saying they'd hate for him to lose it.
Bonnaire convinces the Musketeers to let him and his wife have a brief conjugal visit in an upstairs room. Athos has the explorer give his world that he won't try to escape and Bonnaire freely gives it. The Musketeers then stand guard outside the room and listen to Bonnaire's cries of passion... unaware that he's faking them while Maria picks the lock on the rear door. She gets it opens and tells her husband that Thibault is waiting with a wagon in the alley outside. After kissing Maria goodbye, Bonnaire drops into the alley and boards the wagon. Thibault drives away but when Bonnaire checks on the driver, he discovers Porthos sitting next to the man. The Musketeer orders Thibault to jump and Aramis rides up to escort them on the road to Paris.
Milady meets with Richelieu in his office and tells him that the Spanish spies are after Bonnaire. She then asks for some personal time off, and Richelieu warns her that she is his creature... but grants her request anyway. The cardinal suspects that she plans to go after Athos again and warns her not to let her personal vendetta interfere in the King's plans. Milady assures him that it won't.
Athos and D'Artagnan ride up to the wagon and they all continue through the countryside to Paris, avoiding the main roads. When D'Artagnan wonders why they're hauling the wagon, Athos explains that Bonnaire hopes to buy favour with Louis using gifts that he brought back, and figures that it's best to humor the man. Aramis rides up from where he has been covering their trail and informs them that the spies from the tavern are following them. The Musketeers drive the wagon into some ruins and draw their swords. However, it's the thugs from the tavern, accompanied by several of their friends, who attack them. Porthos stays back to guard Emile and one of the attacks cuts him in the shoulder from behind.
As Aramis holds off Porthos' attacker, an older man, Paul Meunier, steps out of one of the ruins. He tells the Musketeers that he has no grudge with them and advises them to leave. Bonnaire introduces Meunier as his business partner, and the merchant explains that he has been waiting eight years for Bonnaire to return with the promised goods. The explorer insists that the Musketeers took him into protective custody before he could deliver Meunier's promised goods. The Musketeers refuse to hand Bonnaire over and Athos assures him that they will take his complaints to Richelieu and get him reimbursement. Meunier accepts Athos' word but is confused when Athos tells him to keep his spies at a distant. The merchant assures him that the Spaniards don't work for him, but warns that Bonnaire has other enemies.
Aramis examines Porthos' wound and warns the others that their friend could die without prompt medical treatment. He offers to stitch up the room once they find a place to rest, but Athos insists that they have to move on. Aramis grabs him and asks if he cares about their friend. Hesitating, Athos finally says that he knows of a shelter nearby and tells them to mount up. As they do, D'Artagnan wonders why Athos didn't mention it to them in the first place.
The Musketeers take the wagon through a nearby village and the inhabitants look suspiciously at them. One of them, a blacksmith named Remi, recognises Athos and goes back into his smithy. Meanwhile, the musketeers continue on to an abandoned manor house and Aramis makes Porthos comfortable. Athos admits that he used to own the house when he was the Comte de la Fere, before he joined the Musketeers. Aramis is duly impressed and has Athos punch Porthos unconscious so he can start stitching.
In Paris, Richelieu goes to see Louis, who is obsessed with a model of a Dutch merchant ship. Louis wants to purchase a fleet of the ships for France but Richelieu warns that the cost would be prohibitive. Eyeing the young king warily, Richelieu reminds him that the Spanish court has complained because Bonnaire broke their trade pact and is establishing colonies for himself in the name of France. Louis points out that some might consider Bonnaire a patriot but admits that they will have to punish the explorer to maintain good relations with Spain.
At the manor, Aramis saves Porthos' life while Athos walks through the hall to the study.
Five years ago, Milady is in the study looking out the window. She offers Athos a blue forget-me-not that she plucked from the garden outside.
Porthos wakes up and Bonnaire talks to the Musketeer about his dream of owning a plantation in San Christophe. He suggests that Porthos might join him there and the wounded Musketeer says that he'll think it over. Athos comes in and says that they'll be leaving for Paris in the morning, and Bonnaire mockingly says that he should enjoy staying in his former home. Ignoring him, Athos goes to the master bedchamber.
Athos and Milady make love on the bed, enjoying each other's company.
Disgusted, Athos turns away from the bed.
The next morning, D'Artagnan finds Athos in a portrait gallery. One of the paintings, of a woman, has had the face cut away and Athos suggests that robbers broken in and defaced it. D'Artagnan notices a portrait of a younger man bearing a resemblance to Athos, and the elder Musketeer explains that it is his departed brother, Thomas.
Five years ago, Athos and Remi take Milady out into a field and a priest performs the last rites for her.
In the study, Porthos watches as Bonnaire looks at some papers from the map case. He asks to take a look but Bonnaire hastily puts them away, saying that his eyes have grown tired. Outside, D'Artagnan calls a warning as he and Aramis spot Maria riding toward the manor. The Musketeers and Bonnaire go to greet her, and she says that the spies attacked her on the road and wounded her. When D'Artagnan goes to help her down, Maria draws a gun on them all and orders them to surrender their weapons. Once they do, Bonnaire mounts up behind her and Maria tells them that she was her husband's scout in the jungles of Brazil and tracked them to the manor.
As the Bonnaires ride off, the Musketeers mount up to pursue. Athos tells Porthos that he's in no condition to ride and orders him to stay back. Meanwhile, the two spies intercept the couple in the forest and one of them opens fire, killing Maria. The Musketeers ride up and Aramis shoots the spy, while Bonnaire begs his wife's corpse for forgiveness and then gallops off on her horse. D'Artagnan rides after him at a slower pace while Aramis questions the dying spy, who mutters a few words in Spanish.
At the manor, Porthos realises that Bonnaire has left his map case behind.
Bonnaire's horse finally stops, worn out from its gallop. D'Artagnan easily catches up to the explorer and orders him back to the manor. When they arrive, Porthos attacks Bonnaire as soon as he sees him. The other Musketeers hold him back and Porthos tells them that Bonnaire is planning to build a slave ship. Bonnaire admits that it's true but insists that it's merely business, and Athos admits that slavery isn't a crime. Porthos describes what he knows of slave ships, and how the prisoners have to be shackled so they won't commit suicide rather than face starvation and sickness. Aramis assures his friend that Bonnaire will face justice from the king once they deliver him to Paris.
Aramis and D'Artagnan go to get Maria's body while Porthos watches Bonnaire dig a grave for his wife. Disgusted, the Musketeer asks Bonnaire what it feels like to buy people, and the explorer defends himself, saying that he gives his slave a better chance than other slavers do and that the Africans he buys and sells have no other option. The others return with the body and Bonnaire defends riding off, saying that he owed it to Maria's courage. He begs his wife for forgiveness, saying that she deserved a better husband, and Aramis utters a brief prayer.
After drinking, Athos goes to the spot where Milady died.
Athos watches as Remi strings up a noose. Once the priest is finished with the last rites, Athos nods to the blacksmith and then rides off, while Remi hangs Milady.
D'Artagnan checks on Athos, who says that there's someone he needs to meet in the village. He tells D'Artagnan to keep Porthos away from Bonnaire and make sure the explorer gets to the king safely. Once Athos rides to the village, D'Artagnan relays their leader's orders to the other. He and Porthos both want to follow their friend, but Aramis advises them to trust Athos to handle his own affairs.
Athos rides into the village and goes into the smithy, and finds Remi dead, his throat cut seemingly by his own hand. Shocked and guilt-stricken, Athos rides back to the manor and drinks himself into a stupor. That night, he wakes up to the smell of smoke and follows it to the study. Milady has set the drapes on fire and knocks Athos down. He wonders how she can be alive when he saw her hang, and Milady reminds him that he rode off and left the execution to Remi. She had seduced the blacksmith beforehand and he spared her, and then spent the last five years half-dead with guilt waiting for Athos to return and find out his crime. Milady boasts that she merely put Remi out of his misery by killing him and prepares to dispose of Athos as well. He reminds her that she killed his brother Thomas when he found out that she was a criminal, and Milady insists that she did it for the two of them. Consumed with guilt for ordering her execution, Athos begs his former wife to kill him and as she raises the knife, she finds the locket around his neck that she gave him.
Before Milady can strike the final blow, D'Artagnan calls to Athos from outside. Milady flees and D'Artagnan gets a glimpse of her before running into the burning manor. He pulls Athos to safety and the older Musketeer explains that the woman who tried to kill him was the wife he ordered dead five years ago. D'Artagnan wonders if the woman is a ghost and Athos assures him that she is all too real, and wonders what he will do now after five years learning to live without her.
In Paris, Porthos and Aramis arrive with Bonnaire, who demand a new set of clothing for when he meets with the king so he can have some dignity. Aramis points out that he's a slave trader and Bonnaire swears on the spot to give up the trade. Neither Musketeer believes him and they take him to Richelieu.
Milady goes to the Bonacieux household on the pretense of wanting to buy a dress. Bonacieux is out and Constance greets the woman. Milady comments on Constance's beauty and asks after D'Artagnan, wondering if he is Constance's lover. Constance denies it and Milady says that she is an intimate friend of the young Musketeer and has a maternal interest in him. She introduces herself as Milady de Winter and says she will return later for the dress. Constance suggests that she stay away but Milady informs her that Bonacieux needs the money and she's willing to pay. She compliments Constance on her beauty once more and leaves.
As Athos and D'Artagnan arrive in Paris, they spot the second spy and Athos tells D'Artagnan to find the others and to not say anything about the woman at the manor. He then rides off after the spy while D'Artagnan returns to his apartment to change his clothing. Constance comes in and tells him that Milady de Winter asked after him. D'Artagnan has no idea who the woman is until Constance describes her, and Constance admits that the woman frightened her.
The Spanish spy goes to a rooftop outside of Richelieu's office and prepares to shoot Bonnaire. However, Athos finds and captures him with ease. Meanwhile, Bonnaire meets with Richelieu and the cardinal demands to know what he's been doing, establishing plantations in the name of France throughout the New World. Bonnaire assures Richelieu that the plantation business is very profitable and the cardinal tells the explorer to explain his plans as if his life depends on it.
A short time later, a shocked Bonnaire emerges from the cardinal's office to where Porthos and Aramis are waiting for him. They wonder how the king plans to have him executed for breaking the trade pact, and Bonnaire explains that he and Richelieu are now partners. Together they will build a series of plantations throughout the Antilles. Disgusted, Porthos realises that Bonnaire will be shipping more slaves than ever before. When Aramis reminds Bonnaire that he swore to give up slave trading, the explorer says that circumstances forced him to adapt.
Later, Porthos and Aramis meet D'Artagnan and Athos at a tavern and share a drink. Porthos wishes that the spies had killed Bonnaire and regrets saving the man. Athos explains that after interrogating the remaining spy, he's learned that the Spanish King sent them to kill Bonnaire for breaking the trade pact.
That night, Bonnaire is back in Le Havre buying drinks at the tavern. Meunier is there with his men and the merchant reminds Bonnaire that they had a deal. Bonnaire warns that he is under Richelieu's protection now, but Meunier sends the thugs to kill him. However, the Musketeers intervene again, holding off the killers. Disgusted, Porthos says that they should let Meunier have Bonnaire. When Aramis insists that it's their duty, Porthos attacks him. As the thugs close in, Athos tells D'Artagnan to take Bonnaire to a waiting ship. D'Artagnan does so and Bonnaire invites the Musketeer to visit him in the Antilles sometime. However, when he boards the ship he's shocked to learn that it's a Spanish vessel and the remaining spy is there to greet him.
Back at the tavern, D'Artagnan joins his comrades and they share a toast with Meunier. Porthos and Aramis are satisfied that their fake fight was convincing enough to get Bonnaire to flee. Athos gives Meunier the key to Bonnaire's warehouse and warns him that he should remove the explorer's goods before Richelieu sends his men to get them. After the merchant leaves, the Musketeers outside and Porthos thanks his friends for stopping Bonnaire's slave trade. As he leaves with Aramis, Athos admits to D'Artagnan that he wishes all evils could be easily fought... unaware that Milady is watching him from the shadows.