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.

Mary and Francis play up their romance after an English envoy expresses doubts about their union. Elsewhere, Catherine seeks to eliminate an individual who's aware of her scheme to destroy Mary.

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.

Francis observes the growing bond between Mary and Bash, so he turns to Olivia. Meanwhile, Catherine wants to ensure that whoever threatened Mary is captured; Bash seeks to repay his debt; and Kenna tells her friends about her affair with Henry.

Mary and Francis are taken hostage when the castle falls under siege, and Catherine makes a shocking suggestion in an effort to save them. Meanwhile, Diane reveals to Bash that she's plotting to have him declared legitimate.

Nostradamus shares with Mary his prophecy about a death, which leads Mary to make a big decision concerning Francis. Meanwhile, Clarissa gets brazen as she pursues her goals; and Kenna tells Catherine about Diane's intentions regarding Bash.