In the series finale, Mary changes the course of history with a decision she makes, and her perilous move for survival causes her to have a stunning realization. Meanwhile, Elizabeth steadies herself for war and becomes the instigator in Mary's demise.
Childbirth becomes dangerous for Mary and leaves her and her unborn child's lives in jeopardy. Meanwhile, Elizabeth discovers that someone in her inner circle was the cause of death of someone close to her.
Darnley teams up with John Knox to strip Mary of her throne, and this plot will leave a friend of Mary's dead. Meanwhile, Elizabeth must banish Gideon from court; and Charles takes a bold stance against his brother Henri.
An earthquake hits Scotland, and Mary faces claims she isn't coming to the aid of her people. Meanwhile, Elizabeth takes drastic measures after she is discovered with Gideon; and Charles make a decision that backfires.
Mary and Darnley are a odds as they put their plan to seize England into action. Meanwhile, Elizabeth's conflicted about her feelings for Gideon as she tries to find a Catholic to wed; and Catherine's son Henri attempts to claim the throne from Charles.
Mary strategizes ways to take over the English throne. Meanwhile, Elizabeth struggles to find an appropriate suitor to marry; and James gets caught up in a scheme orchestrated by John Knox and has to deal with the tragic aftermath.
Mary and Darnley wed in a lavish affair, which proves to be a successful decision in regard to her political standing. Meanwhile, James makes an interesting discovery about Greer; and Elizabeth frantically tries to salvage England's power.
A tragedy occurs, pushing Mary and Darnley apart as their wedding looms. Meanwhile, Mary is forced to enlist help from Catherine; and a traitor to Spain petitions Elizabeth to support an expedition to the New World.
Mary's marriage to Darnley hangs in the balance when Darnley receives a surprise visitor. Meanwhile, Elizabeth's plans backfire; and Catherine and Narcisse find Charles and attempt to convince him to return to the castle.
Mary's potential marriage to Lord Darnley has political consequences she must face. Meanwhile, Charles vanishes, leaving Catherine to handle things; Elizabeth helps Gideon; and an intriguing proposition is made to Claude.
Lord Darnley must prove himself to Mary in order to get her to accept his marriage proposal. Elsewhere, Catherine and Narcisse have to do damage control after rumors spread regarding Charles' eccentric behavior.
Lord Darnley surprises Mary with a visit to Scotland, which allows her to see more to their union than just a political bond. Meanwhile, Elizabeth attempts to acquire allies; and Catherine wants Claude to focus on securing a new husband.
Charles behaves erratically, but Catherine covers for him as best she can. Meanwhile, Gideon wants Mary to see what life could be like if she prioritizes love over country; and Claude and her sister Leeza become unexpected adversaries.
Mary learns there's a traitor within her inner circle of advisers, and she's determined to unmask the guilty party. Meanwhile, Elizabeth seeks Narcisse's help as she tries to expand her power; and Catherine is desperate to reclaim some of her lost power.
In the fourth-season opener, Mary tests her brother after he meets with John Knox, because she's unsure about whom she can trust. Meanwhile, Elizabeth schemes to sabotage Lord Darnley's relationship with Mary.