The relationship between Mary Stuart and Sebastian. The two seem to have taken an instant liking to each other particularly Sebastian to Mary as he reassures her she is not alone. Both have grown closer soon after learning of Nostradamus' prophecy.
They first laid eyes upon each other in the Pilot when Mary arrived at court again. Their eyes met and they seemed to be quite intrigued by each other, her being new to the court and him having a scandalous past (he is the illegitimate son of the King and his mistress).
When Mary's dog Stirling ran out into the surrounding woods of the castle, she ran after it, almost entering the woods. Bash caught her and warned that young girls, royals, queens don't leave the castle alone and told her not to go into the woods and that he would find her dog for her. Immediately, he showed concern for her safety and comfortability at the castle. When he teased her about her life at the convent with the nuns, she said that he was cheeky. Because of this raport, they established an easygoing, almost flirtatious relationship. When Mary danced at Elisabeth and King Philip II's wedding, she and Sebastian paused and gazed into each others' eyes. Caught up in the moment, the gaze held and something- not nameable- passed between the two. From that moment on, their flirtatious relationship turned into a deep and admirable respect for each other, built on hints of romance.
He later on found Mary's dog, Stirling and, proving his desire to make her happy and keep her safe, returned him to Mary. Upon his arrival, he noticed that she was distraught and sought to comfort her. He told her that she was not alone and he was not talking about her friends, but him.
In Snakes in the Garden, Mary went to Bash and informed him that Colin was the only one who knows the identity of her biggest threat at court, convincing him to help her locate the lost boy. He deduced that Colin may ended up in the woods and that the guards, who exited from the south keep, wouldn't venture into them since they were afraid. Even though he knew the woods were incredibly dangerous, he went anyway in search of Colin for Mary's sake. Also in the episode, Bash cheered Mary up when she was down by telling her that her presence at court "brings light" in dark and dangerous times.
In Kissed, after the six companies lead by Bash were slaughtered and Bash was gravely wounded, Mary felt responsible and guilty for the injuries Bash sustained but for the good of her country and upon Francis's request, Mary broke off her engagement and alliance to France. In doing so, Mary did the very thing Sebastian put his life at risk to stop. Bash felt that Mary should stay engaged to Francis because he loved Francis, despite their differences, and liked Mary and wants them to be happy together; by leading the six companies to Scotland's borders Bash was trying to save the alliance and happiness of two people he cared for. The irony of Bash's relationship/possible romance with Mary is that the only way Bash will be able to see her (much less be closer friends) is if she kept the alliance with France and the alliance with France was the bethrothal to his half-brother Francis.
In A Chill in the Air, in the market, Bash flirted Mary for while, then Mary asked if he flirted with everyone, he replied "Absolutely everyone". Lola then said that Bash had feelings for Mary but Mary objects, saying that Bash himself confirmed that he flirted with everyone. When Olivia came to the Court for help, she ran into Francis arms. The action piqued Mary's curiousity, and asked Bash who she was, which Bash replied that she left a few months before her arrival and revealed that Olivia broke Francis' heart. After the arguement with Francis, Mary approached Bash and asked him for the bottle of wine he was drinking. Bash smiled when Mary grimaced and said that he should tell her to slow down. Then she told him all of her sorrowness while she was drunk. Mary inquired if he thought she was overreacting, that she was boring him but he answered no, saying that it was unfair because Francis had Mary so why would Francis ever look elsewhere. They gazed at each other then Mary kissed Bash. She regretted immediately and apologized Bash for the kiss, that she shouldn't have done that. Bash agreed, said that he should have. Bash then kissed Mary deeply, putting a hand on Mary's cheek. She quickly pulled away then stopped the kiss. Francis saw the whole scene. She stood up and said that they shouln't have done that, that she was angry. Then because of the horn sound, she quickly apologized and ran away.
In Chosen, when Mary woke up, she saw a necklace, thought that Bash gave it to her. When she saw Bash, she came to him, was wanting to discuss about the kiss and the necklace. When Mary mentioned the kiss, Bash had a smile on his face said "I remember it." Mary smiled a bit too, she said that she was drunk and Bash was a little drunk too but she wasn't sure. He said that he regret that Mary was engaged to Francis, whom he considered as a friend and who thankfully, Bash thought, didn't know about the kiss. Mary shared that Francis and she had been distanced since the argument, and she had to make things up with him. Bash understood but he didn't know why they were still talking about it. Then she said because the necklace that she thought it was his gift. He was confused, because he didn't know anything about any necklace. She realized it wasn't him and said that it was in her room this morning. He asked if she thought that he snuck into her room and Mary said that she didn't know what to think after the kiss and the argue with his brother. Then Mary showed Bash the necklace. Immediately, Bash grew concern because it was the pagan's necklace, he said that he'd seen the necklace before but she ran to Francis before he had a chance to explain. Then he asked Mary and Francis to have a talk, since it was important. Then he explained the debt that he had to the pagan and he assumed that Mary's been chosen. When Mary was going to ask the servant, Bash called her back and apologized. Then he told Francis that he wouldn't have took those bodies down if he 'd known that Mary would be chosen. Bash went to the prison to choose a thief in exchange for Mary. Then he came back the Court and announced to Francis and Mary that it's done, the debt had been paid, that Mary is safe. Mary grew concerned, asked him what did he do, worried that he might killed somebody for her. Francis showed jealousy between those two. He accepted for Mary to be with someone else, even if that meant to let her go but anyone except Bash. He asked Francis if it was warning and Francis confirmed. Then Bash and Mary gazed at each other with worry.
In Fated, Sebastian didn't like that his mother still wants him King to have Mary. Later, Sebastian was combing a horse. Mary and he had a talk about Nostradamus' visions he admit he was the lion, a symbol chosen for himself like all other bastard-borns did, that fought Tomas, the dragon in the field of poppies. At the end of the episode, Bash and Mary left the castle while Francis tried to stop them, but he was too late.
In For King and Country Mary and Bash leave for different reasons. They are both brought back to Court, and while Bash is locked up, Mary comes up with the plan for her and Bash to wed. For now they are unofficially engaged.
In Sacrifice a pregnant girl comes to court being accused of stealing. Mary assumes that it’s Bash’s child but Bash tells her that Isobel, the girl, is his cousin and her father was a traitor, so nobody could know about her. They both leave with Isobel and eventually help her give birth to a daughter. Isobel dies right after the birth. Mary tells Bash that she wants him to have a choice. She thinks that in his heart he doesn’t want to marry her. She also tells him that she thinks it’s noble that he’ll always put his family first.
When they pay their respects to Isobel, Bash tells Mary that he’s not Francis. His duty will never be to some country, some land, some throne. If he’s married to her, she’ll be his family. He’ll be in it for her but only her. Then they kiss.
In Inquisition, Mary and Bash meet with the woman who is taking the baby. Mary and Bash finding out that the pagan mark has not disappeared from the baby's foot. A guard informs them that the King has returned to the castle. Catherine walks in when Mary and Bash are sending off the baby with the wet nurse's mother. The plan is to send the baby to the convent. Catherine notices the pagan mark. She warns Mary about the consequences of Bash's link to Pagans. Mary might very well end up on Bash's adjacent stake. Mary tells Catherine that Bash will be at her side when the Queen is executed. Later, Bash and Mary are told that the Queen and her men know about the baby's location. They try to reach the place before the Queen's men. However, they did not reach early enough. The Queen's men reach the place before the two could take the baby and the wet nurse's family to some place safe. Bash and Mary are hiding in a closet when Queen's men come knocking. They are unable to find Bash and Mary. However, they take the baby with them.
The wet nurse's mother tell Mary and Bash about Queen Catherine giving birth to a child, with a scar on her face. She also tells them that the child might have been the lover's child, and not King Henry's. Mary and Bash take her to the King and she tells him, everything. The King figures out that the baby must have been of Richard, as that kind of mark runs his family. He confronts him and sentences him to death. Catherine pulls out the last trick to save herself. She shows a pagan ball to King Henry and tells him about Diane's being a pagan meaning Bash is also a Pagan. The guard saw Catherine find the pagan ball from Diane's drawer. When the King asks if the guard if he really did witness this, he confesses that he did and Henry kills him and the second guard declares that he did not witness anything. Henry reveals to Catherine that he knew about Diane's past, as she told him about it. He tells Bash that a King should cut any threat like a weed. Mary tells Bash that her heart is open and kisses him. She does not expect him to be Francis, and conveys the same to him. Bash promises to cut down anyone or anything that harms Mary.
In Royal Blood, Mary meets Bash and voices her concern about Henry Jr. and Charles. He has made a plan to go to a Frost Fair in the town with Charles and Henry Jr. The three, however, could not make it to the fair because the carriage in which they are travelling is attacked. Bash and Mary fear for the safety of the two princes, but they differ when it comes to, how to protect them. Bash wants to send Charles and Henry out of France, while Mary wants to keep an eye on them in the castle and believes that Bash wants the children to leave is because they remind him of Francis.
Bash arranges a frost fair for the two Princes inside the castle. Mary finds the gesture worth giving Bash a kiss. Later, Charles and Henry disappear. Mary comes to know that it is Bash's doing as he wants to send them away to protect them. Mary is not happy about it. Catherine joins Bash, Mary and the guards to find the two Princes. Bash, Mary, and Catherine find Clarissa. Henry Jr. manages to get away but Clarissa holds Charles at knife-point. Catherine tries to convince her to let Charles go and not be like her mother who is now a condemned woman. Clarissa tightens her grip on Charles but before she could slit his throat, Mary hits her with a rock, killing her. Near the end of the episode, Bash finally agrees that the Princes are safer inside the Castle as they can protect them. Mary wants to marry Bash that day and asks Bash will he marry her. She does not want to wait for the Vatican's approval, legitimizing Bash. Bash gets down on his knee and proposes to her. Mary says yes.
In Consummation, Mary chooses Francis to wed and Bash is devastated so he doesn't attend the marriage. Henry II forced Bash to watch Mary and Francis' consummation in the end.
In Dirty Laundry, after Bash returns to the French Court to have Nostradamus care for Olivia, he sneaks into Mary and Francis' room. He warns Mary that it was Francis who tried to have him killed. Mary doesn't believe her husband was capable of that but Bash presses her. She tells him that it would not change anything. It would change her heart, and if Francis did try to kill Bash, Bash would kill him. Mary cautions Bash to watch his words, that they are treasonous. He tells her for her own sake, she should find out who she is married to. He tells her that if she needs him to put a light in the woods, and he will meet her where they first kissed.
The next day, Mary asks Bash for his help to get through the Blood Wood and he cannot believe that Lola would go there but desperate times call for desperate measures. She also asks that once they get there that he let her go in alone so that they don’t cause Lola any additional embarrassment. Later, Bash asks if Mary will be okay. Mary is not sure. Francis told Mary that he was innocent on the attempt on his life, and she believed him. Mary wonders if Bash should leave, he cannot care for her she says. But she’s there, and he cannot hate her, and knows someone needs to protect her.
In Monsters, Mary becomes upset with Bash for returning to the French Court knowing full well that he could get himself killed. When she arranges plans for him to leave but he refuses until he gets help in rescuing Rowan and rid of the Darkness. By the end of the episode, she attends Sebastian and Kenna's Wedding.
In Higher Ground, Bash finds out that Mary is behind the kidnapping of Catherine and has gotten herself in trouble yet again. Mary asks Bash what is was like when he had to chose a sacrifice to save her life. He could not believe that Mary was going to involve and innocent to clean up her mess. Bash warned Mary of the choices she would make would have dire consequences making Mary remember everything they had gone through with pagan sacrifice. Bash tells Mary she’s incapable of taking an innocent life for the kidnapping of a Queen. He had only taken a life to save hers not out of cold blood. Bash said she needed to be clear in her conscience or she would never live it down. Finally, Bash realized that was why Mary wanted to talk to him.
Mary says she loves both Bash and Francis but she loves Francis more.
Before Mary married Francis in The Consummation, Mary told Catherine that she loved both Francis and Bash but later confessed Francis was always the one in her heart.
Mary goes to Bash for help many times throughout the series.
Bash was the first one to call Mary by her name and not "Your Grace".
Mary stated that Sebastian is cheeky even though he becomes serious at times.
They were engaged due to Mary and Bash's attempt to save Francis from the prophecy.
They are officially unengaged as Mary is now married to Francis.
Bash knows that he can never have Mary because she is his little half-brother Francis' wife.
Bash was forced to watch Mary and Francis consummation to know "what is not his".
Sebastian flirted with Mary when he saw her "for the first time".
In Pilot, Mary and Bash had a moment where they gazed into each others eyes.
Their first kiss happened when Mary was in a drunken state after having a fight with Francis over his first love Olivia in A Chill in the Air. She was looking for a "distraction".
There are "some chances" that Mary and Bash might be together if Bash gets legitimized but after Henry died in Slaughter of Innocence there was no chance of Bash being legitimized.
Mary and Bash most likely have no possibility of getting back together as Bash is married to Kenna and Mary is married to Francis. Because of their marriages, they can never be divorced due to the fact that they are in the 16th century and it would be extremely difficult for an annulment due to the fact they have both consummated their marriages
Bash was willing to dispose of his brothers and sisters, Francis, Elisabeth, Claude, Charles, and Henry III, for Mary even after the prophecy was nullified and wanted to force the Pope's hand to become the next King of France and King Consort of Scotland.