In the upcoming episode of The CW's "Reign" season 4, Mary (Adelaide Kane) learns that Elizabeth (Rachel Skarsten) is planning to stop her wedding.

Titled "Hanging Swords," this week's episode will see if Elizabeth could stop Mary's plans to get married. According to the official synopsis, Mary's marriage to Darnley (Will Kemp) hangs in the balance when he gets a surprise visitor. Catherine (Megan Follows) and Narcisse (Craig Parker) try to find Charles (Spencer MacPherson) and bring him back to the castle, while Elizabeth discovers that her attempt to stop Mary backfired.

In the promo released by The CW, Elizabeth appears prepared to invade Scotland. She will take all of Mary's actions as an act of aggression, resulting in further escalating tension between them. Despite being advised that she cannot take over Scotland without any reason, Elizabeth is determined to do everything in her power to get back at the Queen of Scots. Meanwhile, it seems that Mary has already made up her mind in marrying Darnley to save her people.

The previous episode titled "Love and Death" saw Elizabeth send Ambassador Richards (Philip Riccio) to bring Darnley back to England. The plan failed because Mary had been warned by the "Loyal Watchman" about an assassin who was out to kill Darnley. Fortunately, he was able to defend himself against the killer.

As Darnley and Mary's relationship reached a new level, Keira (Sara Garcia) went to Scotland in an effort to reunite with Darnley. Unable to cope with the idea that Claude (Rose Williams) and Luc (Steve Lund) are married, Leith (Jonathan Keltz) demanded payment from Narcisse to leave the country for good. Claude asked Leith to meet her after a year she has given Luc an heir. Also, in the episode, Elizabeth tried to help the daughter of Gideon (Ben Geurens) by bringing in a physician, but he was unable to treat her illness. The episode concluded with Leesa (Anastasia Phillips) promising Spain will invade France after learning that Charles had fled.

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