Catherine and Mary will work together in "Reign" season 3 episode 7, which is titled "The Hound and the Hare." Their team-up will form after the latter happens upon a dark secret being kept by Prince Don Carlos of Spain, who just arrived in France.

However, it won't be easy for Catherine to be on board knowing that what they are up against a man whose territory holds the greatest power in all of Europe. But it looks like she will get past that anyway but danger definitely lurks. As per the promo, this "royal mystery" involves dead bodies.

Sebastian reveals in the "Reign" season 3 sneak peek that killing is a "compulsion" for the prince and Don Carlos is about to satiate his craving for blood and death with Mary, whom he chose as his next target. The clip shows that he was able to corner the queen.

Fans hope that Mary will be able to get out of this trouble safe and sound. The last few episodes of the season have sent viewers in a whirlwind of emotion, especially after the death of Francis. Mary is barely given time to take that in before fully moving on with perils already confronting her from every direction.

On the other hand, in the upcoming installment, Prince Don Carlos won't be the only one inviting mistrust. The new ambassador from English court, Gideon has reached the place as well and his aura seems to elicit something foreboding.

As the synopsis warns, he might not be what he seems. Upon his arrival, "suspicions arise about what his true intentions are" and it looks like he is brewing up something heinous.

To cap up the upcoming "Reign" season 3 episode, fans will Bash is forced to do extreme measures as she confronts the mysterious killer at Greer's tavern.

"Reign" season 3 episode 3 airs Dec. 4 at 8 p.m. ET on The CW.