Mary arrives in France to formalize her arranged marriage to Prince Francis, but complications arise when she meets his attractive half brother and skeptical mother in the series premiere of this historical drama about young monarchs.

A Portuguese prince offers much-needed aid to Scotland if Mary accepts his marriage proposal and break things off with Francis. Elsewhere, Francis' feelings for Mary lead him to put Bash in a risky situation.

Mary has a chance to get out of her engagement, but it requires lying and sentencing a man to death. Meanwhile, Francis and Bash suspect that Tomas may be disguising his real motives.

An ex-lover of Francis' arrives and brings danger with her, so an uneasy Mary turns to Bash for comfort. Meanwhile, Queen Catherine persuades Aylee to snoop on Mary; and Bash lands in hot water with heretics, who accuse him of disrupting a sacred ritual.

Mary's brother visits and asks her to return to Scotland, but Francis is suspicious of his motives. Meanwhile, Penelope takes command as Henry's mental instability progresses; and Lola is skeptical about Julien's marriage proposal.

Mary's uncle arrives at the castle and brings dire news about her mother, forcing Mary and Francis to make a tough choice about priorities. Meanwhile, Henry devises a destructive plan; and Bash and Kenna grow closer as they strengthen their forced union.

Mary hires a mercenary to help her save Scotland. Meanwhile, Francis prepares France to wage war on England, and Leith becomes his ally in this endeavor; and Lola and Julien's bond deepens, but she fears he will betray her.

Mary teams up with Catherine to deal with Henry, whose deranged behavior puts Francis at risk. Elsewhere, Bash resolves to destroy the Darkness after meeting a boy who survived the horror; and a devoted Lola tries to protect Julien.

Mary and Francis take drastic action in response to Henry's madness. Meanwhile, a terror in the woods puts everyone in peril; and Mary makes a bold choice when she discovers Lola is in the midst of a life-threatening labor.