In the third-season opener, Mary faces struggles in Scotland as Elizabeth and Catherine unite against her. Meanwhile, Elizabeth loses focus because she grows tempted by her first love; and Francis' fate remains uncertain.
Catherine is imprisoned when she returns to France, and Elizabeth faces a political backlash for sheltering her. Meanwhile, the fate of Scotland's future forces Mary to make some tough choices; and Lola feels torn regarding Narcisse.
Mary's mother sends her a pressing message to inform her of Scotland's troubles, so Mary seeks help from Francis. Meanwhile, Narcisse becomes an obstacle in Catherine's endeavor to become the regent to Charles; and pressure mounts for Elizabeth to marry.
Mary envisions her future in France by Francis' side as his condition improves, prompting her to contemplate signing away her claim to the throne of England. Meanwhile, Nostradamus returns with more news of Francis' fate.
Mary must assist Catherine in her quest to become regent, even though the cost is high for Mary. Meanwhile, Claude sets out to help Leith marry; and Elizabeth has trouble facing the fact that she may lose Dudley.
Mary unearths a dark secret being kept by a Spanish prince, so she seeks Catherine's counsel. Meanwhile, suspicion surrounds a new ambassador from English court; and Bash's quest to find a mysterious murderer takes him to Greer's tavern.
Mary must marry again in order to help Scotland, whose welfare she has to prioritize over her heart. Meanwhile, Catherine is accused of poisoning Francis, but Mary believes Catherine is being framed by Narcisse, who's angling to become regent of France.
Claude is forced to marry someone selected by Narcisse. Meanwhile, a surprising discovery is made by Elizabeth, and it could alter her reign; and Mary asks Gideon to help her confirm her suspicions about Prince Don Carlos.
Mary's feelings for Gideon are hard to suppress, but she does her best as she seeks political allies. Meanwhile, Elizabeth and Greer face gut-wrenching scenarios; Claude is in danger on her wedding night; and a power struggle over the regency continues.
Mary unwittingly endangers Gideon as she attempts to safeguard Elizabeth, so she then turns to Dudley to help her save Gideon. Elsewhere, Bash searches for the individuals who are responsible for threatening Catherine.
Mary has to raise an army in order to save her throne in Scotland, and she seeks Bash's help. Meanwhile, Elizabeth turns to Lola to assist her in obtaining information about her mother, but this mission faces a setback when Lola gets caught.
Mary arrives in Scotland and must hide her identity in this suddenly treacherous and changed land. Meanwhile, Elizabeth makes a misstep with an insufferable new suitor; and Charles covertly schemes to have his mother surrender her power.
Mary is presumed dead, so Elizabeth seeks her crown with assistance from Lola. Meanwhile, Mary's half brother tries to safeguard her throne from John Knox; and Mary and Bash wind up in the clutches of the intimidating clan leader who killed Francis.
In the third-season finale, Mary must battle Elizabeth in order to secure her crown in Scotland. Meanwhile, Lola is accused of an awful crime and faces punishment; and Catherine finds a very different Charles has returned to France.