Mary's (Adelaide Kane) struggle to survive life in the French court after the death of her husband Francis (Toby Regbo) continues in the upcoming episode of "Reign" season 3.

In the previous episode, Mary did everything that she could to honor Francis' dying wish – to help his mother Catherine (Megan Follows) secure the regency in France in order to protect the throne. Mary even had to dissolve the long-standing alliance between France and her own country Scotland so that the court will start considering Catherine as the caretaker of the French throne until her younger son Charles assumes the throne.

But while Mary and Catherine successfully earned the council's trust in the previous episode, Mary knows that she still has to marry again for political reasons. This time, Catherine returned the favor and gave her support to Mary's future plans.

Before the end of the episode, Mary reached out to the Spanish prince Don Carlos (guest star Mark Ghanimé) to invite him in France and talk about the possibility of having an alliance.

In the upcoming episode, Don Carlos will gladly visit the French court. Yet his dark secret will be uncovered by Mary. This will lead her to seek Catherine's advice in order to know how to handle the truth about the visiting Spanish prince.

The official synopsis for the episode reads:

"When Prince Don Carlos of Spain visits France, Mary uncovers a dark secret he is hiding and enlists in Catherine to advise her on how to handle [it]. When Gideon (Ben Geurens), the new ambassador from English court arrives, suspicions arise about what his true intentions are, while Bash's (Torrance Coombs) pursuit of the mysterious killer comes to a head at Greer's (Celina Sinden) tavern."

Meanwhile, the trailer for the upcoming episode reveals that a possibly fatal threat is looming over the French court.

"Reign" season 3 episode 7 airs on Friday, Dec. 4, on The CW.