Welcome to being a ruling queen, Mary. Men will trust you and die. These are the first, but they won't be the last.
Catherine to Mary about being a Queen
In the Pilot, while planning to celebrate the wedding of her daughter, Elisabeth, Catherine learns from the court seer, Nostradamus that Mary's presence will cost Francis his life. Upon hearing this concerning news, she plans for Lola's lover, Colin, to rape Mary after she drinks wine that has been spiked; this attack would render Mary unable to wed Francis thus saving him from an impending death. However, Catherine's plan failed and Colin was caught in the act when he woke Mary up during the attempt.
In Snakes in the Garden, while celebrating the engagement of her son, Charles, to Madeleine, Catherine learns that Colin who knows about her plot to destroy Mary is still alive and she stops at nothing to make sure he is silenced for good. When Francis finds out that that it was possibly Catherine who planned to have Colin rape Mary, he warns her that if it anything happens to Mary, he will suspect her and she will lose him.
In Kissed, Catherine is informed of another of Nostradamus' visions - this one centers on war and the costs of such events. He says destruction will find its way into the castle.
In Hearts and Minds, Catherine is eager to get rid of Mary the girl who promises bring death to her son, and see her be off to Portugal to marry Prince Tomas, and save Scotland from the English. However, she isn't able to get out of her engagement to Francis quite yet, as Henry feels like Scotland was poached; she learns, though, that there is the possibility of her union with Francis being dissolved. Henry and Catherine need someone with an unassailable reputation to put their name to the words of a prostitute named Judith who claims to have seen Simon boasting about his involvement in the slaughter. After the meeting, Nostradamus walks along with Catherine and hears that she's willing to accept what happened with the ambush, even Bash's potentially fatal injuries, because of the fact that it could push Mary out of the castle once and for all. For her, the ends of the slaughter certainly justified the means, even though she learns that Nostradamus' dreams have only become more vivid in recent days.
In A Chill in the Air, Catherine plans for Olivia to return to the French Court to get Francis into bed so that he will forget about Mary. Aylee is forced to spy on Mary by handing her Mary's letters to her mother in Scotland by blackmail, after Catherine catches her "stealing" Mary's ring.
In Chosen, Catherine scolds Olivia for taking too long of getting Francis into bed. When she discovers that Mary has been chosen for a pagan sacrifice she sets out to find who is responsible. In the throne room, Mary and Catherine call in the castle servants where the former offers them a chance to offer any knowledge they may have about who was responsible for the stag's head. Though Mary initially mentions that this would be a moment free of judgment and that everyone in the castle would suffer more if no one came forward, no one comes forward. Catherine decides to amp up the threat level, telling the servants that she'll burn down their houses if they don't provide information by midnight. Later that night one of the servants comes to Catherine's chambers and tells of seeing one of the royal guards carrying the stag's head, a guard who came into the kitchen for scraps. Though she didn't get to see a face, she noticed that he was covered in blood, meaning that he would have had to change and thus wouldn't be as neat as the other guards. Catherine realizing that her guard Robert is responsible, and a heretic she rushes outside her room looking for this guard, whom she chastised earlier in the day for his lackadaisical uniform, only to find that he's fled the castle without her permission, as she never dismissed him. Robert is apparently found towards the end and severely punished for his crime.
In Left Behind, When the castle is taken hostage by an Italian Count named Vincent; Catherine and Mary must work together to save not just the court, but her children, when the count plans to take Francis and his brothers as ransom. Kenna, and Lola are reluctant to help Catherine until she tells the girls of a story where as an 8 year old child she was kidnapped, held hostage and then almost raped but was rescued by men of God. At the end of the episode Mary asks Catherine if the men were too late to save her (from being raped). It is still unclear whether or not she actually was raped.
In Fated, Due to Henry's sudden decision to have Francis and Mary married, so that they can lay claim to the throne of England. Catherine tells Mary about the prophecy, which causes her to run off with Bash in an effort to save Francis' life. Catherine also gets a visit from Kenna who is still healing from her wounds, and the Queen hears that Kenna was mistaken for Diane and has learned of a treasonous plot to have Bash legitimized. This gives Catherine an idea to get rid of Diane, Kenna and Bash all in one swoop, so she enters Henry's chambers and finds Diane to confront her about the plot though she tries to deny it. Catherine tells Diane that there are two things she can't forgive: betrayal and stupidity of which Kenna is guilty of both, and tries to blackmail her into killing Kenna in exchange for her silence or that Diane leave court and never come back.
In For King and Country, Mary and Bash are found and returned to court, and King Henry suspects that his wife Catherine had something to do with them running off together, and tells her if he finds evidence that she was involved that he'll have grounds to execute her. Catherine later tries to help Mary find a way not to marry Francis. As Mary decides to marry a legitimized Sebastian, Catherine is caught trying to escape and is thrown in the castle tower, as Francis and his brothers lose their birthright to the French throne.
In Sacrifice, Mary goes to the prison cell where Queen Catherine has been kept, she is enjoying all the comforts of life and the list of people serving her has been multiplying. Mary demands to know if the attempt on Bash's life was on the Queen's orders. Catherine does not deny it, but she confirms nothing either, since these accusations are based on no evidence against her. Mary orders that everything should be taken away from Catherine's cell, including food, clothes, tapestry and all the amenities. The guards have to oblige, fearing King Henry's wrath.
Lord Hugo later visits the Queen and updates her on the latest developments. He tells her about the plan to trap Bash and Isobel, together, in the night and then killing them. Lord Hugo says he will return with news of Bash's death.
Lola, Kenna, and Greer are the next to visit her, warning her to stop her plotting against Bash and Mary. Queen Catherine is not the one to back down. She insults the three girls one by one, calling out their weak points. Catherine's taunts and insults gives Kenna an idea. Lola later visits the Queen with two letters in hand in an attempt to blackmail her. One has written evidence that she ordered Bash's killing. The letter is fake but enough to start an investigation that might make the guards crumble and reveal the Queen's hand. The other letter is of the Queen ordering the opening of her gold vault to celebrate Bash as the next King of France. It is in the Queen's hands, which letter she wants to release.
In Inquisition, Still imprisoned in the tower, Queen Catherine is accused of adultery which is a treasonable offense punishable by death. Her cousins Pierto and Francesca come to aid her in her defense by telling her that they have gotten the Pope to refuse to annul her marriage to Henry, and she sets out to save her life, by proving that Bash is a pagan using Isobel's daughter, but the mark has disappeared. Later Catherine and Henry get into a deep, intense discussion and realize that they are both responsible for their once happy marriage being a mess. Meanwhile, Mary and Bash discover that prior to Francis's birth, Catherine had an affair that led to the birth of a child. They inform Henry who realizes that she had had the affair with his friend, Richard de la Croix then decides to order his imprisonment. Catherine encounters Clarissa and confronts Nostradamus believing he has betrayed her. He explains that his father was given Clarissa as a baby and was requested to remove the mark from her cheek; however this was unsuccessful and led to her disfigurement. From this conversation, Catherine realizes that Clarissa is her child. Later that night, after losing her defense, and the support of her family the Queen attempts a murder/ suicide with Mary but is ultimately stopped by Clarissa who saves them both. Catherine is then led back to the tower.
In Royal Blood, Catherine is aided in her escape from the tower by Nostradamus; however as she goes to make her escape to Italy, Nostradamus informs her that both Charles and Henry Jr. are missing. The Queen is horrified upon hearing this news, and refuses to leave until her sons are found. Catherine hides herself in Mary's room as she talks to Bash and only reveals herself once he leaves. Mary argues that Bash is innocent and loves his younger brothers, which Catherine disagrees with. Whilst looking in the Queen's box of keepsakes she figures out that Clarissa has kidnapped the children.
Teaming up with Mary and Bash, the trio discover Clarissa and the boys in the woods. A horrified Catherine watches as Clarissa defends herself by holding Charles at knife point, and threatens to kill him if they don't give Henry Jr. back. Clarissa explains that she planned to kill the boys in retaliation to her mother abandoning her. The boys are freed when Mary seemingly kills Clarissa by hitting her over the head with a rock. After comforting her sons, and spending a brief moment with them Catherine is then escorted back to the castle tower by the guards.
InThe Consummation, Nostradamus informs Catherine that his previous visions had been incorrect which he believes is due to Clarissa's death. Because there has been much bloodshed, deceit, etc. due to these visions Catherine is furious upon hearing this. She encourages Francis to pursue Mary, advising him of her plans to marry Bash that afternoon. Later, Catherine approaches Mary with a letter from the Pope that has the decision of Bash's legitimization. The letter causes Mary to agree to marry Francis, the letter later being revealed to be a blank piece of paper. Catherine is pardoned by Henry for the wedding, as they watch their son and Mary finally tie the knot.
In Dirty Laundry, Henry comes to Catherine asking for her help to conceal the accidental death of the Archduke's sister, Cecilia. Together they: clean up the body, place it back in Cecilia's room, and write a suicide note addressed to the Archduke which explains that she killed herself as she was in love with a man she can never have. The plan seems to unravel when the Archduke reads the note and calls it a forgery as his sister never learned how to write. Catherine comes up with a story of how she walked in on Cecilia and a man in the midst of passion and that the lover must have written the note to cover up the fact that he had in fact killed her. Henry supports her story, and they show the body of the priest that had earlier been murdered by Olivia and state that he was the lover. Later, Henry reminisces with Catherine and names all of her positive attributes and attempts to kiss her; however Catherine turns him down and leaves.
In The Darkness, A horrified Kenna comes to Catherine explaining to her that the King has killed a woman in his bed. She comes up with a story and tells Kenna to keep it a secret. Catherine goes to the King during the party, and he intimidates her into keeping his "illness" a secret between them, as the stability of their kingdom depends on the King. Catherine is obviously worried about her husband, and urges him to see Nostradamus about his headache, but Henry refuses saying he's never felt better in his life, and he'd rather have a doctor from the village.
In Monsters, Catherine plans to cancel the "Queen of the Bean" contest in order to hide Henry's illness from Court and France; however Henry knows about this and accuses his wife of trying to poison him and hide him from their people. At the contest a servant girl named Penelope is crowned Queen of the Bean, and wins the title of "Queen for the day" unfortunately for Queen Catherine Penelope is not as sweet an innocent as she looks, and soon she has Henry eating out of her hand making her think he's in love with her. Though Catherine can see through the servant however, Henry threatens to kill her if she threatens Penelope. Catherine is later poisoned by Henry and Penelope; she collapses in Mary's arms, and later in the throne room a recovering Catherine watches as Henry forces Bash and Kenna to marry.
In Liege Lord, Mary discovers that Catherine and her mother Marie de Guise put a secret clause in her marriage contract, so she sets out to get Catherine back and with Francis' help turn Scotland against her mother. Catherine, later arranges for the slaughter of a dozen Scottish country men and Mary attempts to blackmail the Queen of France. Catherine also has a brief affair with a young Scotsmen who didn't realize until later that he was flirting shamelessly with the real Queen of France.
In No Exit, Queen Catherine finally gets rid of Penelope. When Henry shows further signs of his illness, the worried Queen and Kenna work together to get a Bishop to condemn Henry's behavior which works and he dismisses Penelope from his sight, and realizes that he is going to need his Queen if he is to survive. Catherine takes all of Penelope's jewels, estates etc. and gives them to Kenna and her new husband Bash. Finally when Nostradamus has visions of Olivia's death he sends her to Trinidad alone knowing that Catherine will never let him go.
In Toy Soldiers, Catherine catches Penelope in a lie after she claims to be pregnant with the King's child. The Queen then sends Penelope to the tower as a prisoner, and tells her she is to be sent to Italy to learn to be a proper spy for the Queen, and possibly become a member of her Flying Squad.
In Higher Ground, Queen Catherine is kidnapped by a mercenary named John who was hired by Mary to steal Catherine's money in order to help her mother who is under siege in Scotland. After a furious Catherine is freed, she swears to find out just who is responsible. Mary also asks the mercenary to find a scapegoat for the kidnapping and Catherine's unscrupulous cousin, Hortenza, is decapitated and her head is delivered to Catherine by John right before he frees her.
In Long Live The King, Catherine and Mary work together to poison Henry after his madness reaches a new breaking point, as he becomes more determined to take England despite their forces being depleted after Calais was taken successfully. Catherine and Mary decide against this however when they discover that Henry plans to kill Francis so that he can wed Mary and finally take the throne of England.
Due to Henry's increasing madness in Slaughter of Innocence, Francis, Mary and Catherine plot a coup. At the Joust celebrating the victory of Calais Henry, being jealous of the attention Mary is getting decides to participate in the joust , and is severely injured when a lance goes through his helmet and into his eye/brain. On his deathbed, Catherine and Henry confess their true love for each other, and Henry makes her promise to befriend Diane, then he dies leaving Catherine as the Dowager Queen of France.