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 Elizabeth 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 worrt.
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.
They were currently engaged due to Mary and Bash's attempt to save Francis from the prophecy.
It is thought that Mary never truly loved Bash (partially because their relationship was so rushed, she did not really have time to get know him)as much as Francis particularly because her love for Francis started with her childhood friendship with him, and has thus known him longer. Plus from a psychological point of view she was always told to Mary Francis, for her, love, and her countries safety. With Bash marry was just starting to get to know and love him and did not have a friendship evolve into a relationship, when Francis came back and told her they could be married. Which totally stopped the "Mash" (Mary and Bash) Ship. Plus she felt like she had to Mary him (a second resort) when she could not marry Francis.
Earlier Before Mary married Francis in Consummation, Mary told Catharine that she loved both Francis and Bash and did not know which one to choose to Marry.
They are officially unengaged. Because Mary is now married to Francis. Because she was told the prophecy was no more, Catherine's official firstborn, Clarissa dying by Mary's hand. Thus she chose Francis over Bash because even though she loves both Francis and Bash, She loves Francis more than Bash.
Bash was forced to watch Mary and Francis consummation to know "what is not his"
Even though Mary is with Francis and loves him more. She still cares for Bash's safety and wellbeing. She probably still loves him but like a best friend at this point.