The novel opens in 1058, when Gruadh, or Lady Macbeth, is forty-two years old. She lives in a fortress with her close childhood friends Bethoc, a healer, and Drostan, a monk. Twice a widow, Gruadh misses her second husband, Macbeth. She reflects upon their life together, and her childhood before she met him. She begins to reminisce on her life, and the rest of the novel takes the form of her recollections.
Gruadh was raised by her father, Bodhe, the mormaer of Fife. Her mother, Ailsa, died when she was young, although she remains connected to her mother through Celtic magic and divination, which Ailsa practiced but died before she could pass on to her daughter.
Gruadh spent most of her childhood in the fortress Dun Elgin, learning how to read and write, how to run a household, and how to wield a sword. Although reading, writing, and domestic service are all standard lessons for young noblewomen, Gruadh convinces Bodhe to allow her to learn to defend herself after she is the victim of two kidnapping attempts, the first by a man named Crinan (although he denies it), and the second by Thorfin Sigurdsson. Both men try to kidnap her to marry her themselves or to marry her off to one of their relatives, since Gruadh comes from an important royal lineage. To protect her from future abductions, Bodhe marries her to Gilcomgan mac Crinan, the mormaer of Moray, a neighboring province.
Gruadh is initially unhappy about this match. Gilcomgan is older than she is, and he famously became mormaer of Moray only after killing his uncle, Finlach. However, she warms to him after their marriage, and eventually becomes pregnant with his child. However, before she gives birth, she receives word that Macbeth, a nobleman from Moray whose father, Finlach, Gilcomgan killed, has killed Gilcomgan as revenge. Macbeth, who Gruadh has met a few times previously but disliked (but also kissed once), arrives at her home and forcibly marries her the day after she is widowed.
Gruadh resents Macbeth and refuses to accept him as her husband or as the rightful mormaer of Moray, even as her household and the people of Moray quickly warm to him. However, with time, she begins to see the good in Macbeth. They become a strong team, whose ambitions are matched, and whose combined bloodlines give Macbeth a strong claim to the Scottish crown. Macbeth is out for revenge, for he resents the family line currently in power, who allowed his father to be murdered. Gruadh, too, sees taking the throne as revenge after her father, Bodhe, is killed by King Malcolm or one of his agents. The couple feel that it is their blood right to be king and queen of Scotland, and their duty to their prestigious family heritage and murdered relatives to install themselves on the Scottish throne.
King Malcolm eventually dies, and his grandson Duncan, takes over. Duncan asks Macbeth to serve as his general, and he consents. After years of ill-advised wars, which Macbeth had cautioned against, and a poisoning attempt by Duncan, Macbeth rises up against the king. The two meet on the battlefield with extensive armies, but fight for the crown in one-on-one, hand-to-hand combat. Macbeth wins, and wins the throne.
Macbeth and Gruadh rule Scotland for almost two decades. Although neither their reign nor their marriage is perfect—Macbeth has to deal with frequent threats from Duncan’s young, vengeful son Malcolm mac Duncan, and Macbeth and Gruadh are unable to conceive healthy children—Gruadh reports that Macbeth is a peaceful, wise, beloved ruler who does his best to prepare Scotland for the future.
Macbeth is eventually murdered by Malcolm mac Duncan. He and Gruadh crown Gruadh’s only living child, her son from her first marriage, Lulach, in the minutes before Macbeth dies. Lulach reigns as King of Scotland, but Malcolm mac Duncan also claims the throne, and the two engage in constant warfare.
The novel ends as Lulach prepares for battle, and Gruadh, tired of fighting and politics, prepares to retire to a cabin in northern Scotland, where she will live as a widow and study divination and magic.