Like a well-played game of chess, the pieces are all finally in place for a showdown between Queen Elizabeth of England and Mary, Queen of Scots, in the season 3 finale of "Reign."

The past few episodes of "Reign" saw Mary (Adelaide Kane) trying to get back to her homeland. Her journey to Scotland was not without peril; as her ship sank, she was almost killed by a rival clan and the monarchy almost dissolved. Mary's return to Scotland was timely, as her cousin Elizabeth (Rachel Skarsten) sent her troops to invade the country and would not recall them even after it was revealed that the Queen was still alive.

Things will be coming to a head in episode 18 of "Reign's" third season when Mary's brazen move to defend her Scottish crown results in a war with Elizabeth. To make matters worse, Mary's confidant Lola (Anna Popplewell) is accused of a heinous crime and has to face drastic consequences. Meanwhile, Catherine (Megan Follows) finds that her son Charles (Spencer MacPherson) has changed, a result of his paying the price for his mother's relentless quest for more power.

The stakes have never been higher for Mary, and the Queen of Scots has never looked more fired up as she does in the trailer for the upcoming episode, "Spiders in a Jar." The trailer starts with a shot of Mary looking out at a picturesque valley. She's then seen telling the court that she has "returned home to lead this country to greatness." But one man in particular doesn't seem convinced. Mary asks him to help her unite her people, but she also warns him that if he won't, then she hopes that "God and your Queen have mercy on your soul."

The clip also shows a pensive looking Mary gazing at the throne room before the scene transitions to one of her talking to the people and exhorting them that this is their land. She tells them that the English "might spill our blood, but they will never take the Scotland that pounds in our veins." The people seemed impressed and taken by her impassioned speech. However, Bash (Torrance Coombs) seems worried.

While the season's final episode hints at a war between the two formidable Queens, it's unlikely that it will be shown. Fans are theorizing that the big battle might be used instead to open season 4. However, it has also been speculated that Bash will either be killed or wounded in the finale as a way for the character to make his exit. Coombs is set to leave the show at the end of the season to play the lead in ABC's new series "Still Star-Crossed."

Episode 18 of "Reign" season 3 will be aired on The CW on June 20 at 8 p.m. EDT.