Mary's (Adelaide Kane) commitment to her upcoming marriage is put to the test in the upcoming episode of "Reign" season 4.

Titled "Unchartered Waters," this week's episode will explore a complication involving Mary and Darnley (Will Kemp). According to the official synopsis, a horrible tragedy drives Mary and Darnley apart as their wedding day looms. This forces Mary to seek advice from Catherine (Megan Follows). Elizabeth (Rachel Skarsten) crosses paths with a traitor, who asks her to support a journey to the New World.

The CW released a new promo which starts off with Mary and Darnley having a heated argument. "Mary, if I'm to be your king, then I must be your partner, too," Darnley tells his fiancée. "Partner?" Mary angrily asks. "You have lied to me. How do you expect me to treat you?" The next scene cuts to Mary meeting Bothwell (Adam Croasdell) again and sharing an intimate moment with him. At the end of the clip, Bothwell tells Mary that they "make good partners."

Over the show's four seasons, Mary has had several love interests who all have played a major part in her life. First on the list is Francis (Toby Regbo), followed by Bash (Torrance Coombs), Gideon (Ben Geurens) and Darnley. In the episode last week, "Reign" introduced a new character named Lord Bothwell, who will eventually become Mary's new object of affection. While Mary is currently engaged to be married to Darnley, it seems that Bothwell will throw a wrench in their relationship.

Mary first met Bothwell during a poker game when she was attempting to negotiate a trade for lands under his ownership. Reports say that more scenes between the two characters can be expected in the next few episodes. It is also confirmed that Bothwell will eventually become the Queen of Scots' final husband on the show.

The fourth and final season of "Reign" airs every Friday at 9 p.m. EDT on The CW.