The penultimate episode of "Reign" is titled "Long Live the King." King Henry's madness is set to escalate in this Episode 21 of "Reign," which will put the life of Francis in grave danger. The preview clip of the new episode indicates that Mary and Queen Catherine will team-up to kill King Henry.
"You and I are going to kill the King of France," Queen Catherine can be seen saying to Mary in the preview clip.
In the previous episode, Francis was leading his men into battles against English to gain the control of Calais. The preview clip of "Long Live the King" suggests that he is back in the castle, but King Henry has a new plan as he is fixated on claiming England. This time Queen Catherine will be determined to stop the King from putting her son's life in danger, again. Since it is the question of Francis's life, Mary will be on Queen Catherine's side.
Meanwhile, Bash will leave for the woods to finish the Darkness, after he finds a boy who has managed to escape the Darkness. While Kenna will be dealing with Bash's absence, Lola will find herself in a dire situation after she tries to protect Julien. The synopsis of the episode suggests that Lord Julien will be revealing the truth about his motives.
In addition, it is likely that Leith returns to the castle with Francis. In the previous episode, Francis promised to bestow him with a title and give him some property, as Leith had saved his life. Greer might find herself in a big dilemma seeing Leith back as a person of equal status. She is engaged to Lord Castleroy and the man has given her no reason to break the relationship. But, she is still in love with Leith. It will be interesting to see how she will handle the new development.
Official Synopsis of "Reign" Episode 21 "Long Live the King": "Mary and Catherine form an unlikely alliance when deciding how to deal with the increasingly manic King Henry, which puts Francis in jeopardy. Bash finds a boy who lived to tell about his encounter with the Darkness, which makes Bash determined to end it for good at any cost. Lord Julien reveals the truth about his motives, and Lola, blinded by love, finds herself in a dire situation after she attempts to protect him."
Jeremiah Chechik has directed this episode, while Wendy Riss Gatsiounis and Drew Lindo have written it.