The Idylls of the Queen is set in the Great Britain of King Arthur, as portrayed by Thomas Malory's classic Le Morte d’Arthur. It expands an incident in Malory, in which the Queen is accused of murder, into a complex mystery novel mingling the genres of historical mysteries and Arthurian legends.
The Irish knight Sir Patrise is poisoned at a dinner party given by Guenevere and Sir Mador, the dead knight's cousin, accuses the Queen of the murder. Her fate is to be determined through trial by combat. Unfortunately for her, the best Knights of the Round Table were all present at the dinner, which disqualifies them from championing her, and the mightiest of all, Sir Lancelot, has gone missing.
Guenevere's only hope is Sir Kay, the first-person narrator of the tale. Kay suspects that Sir Patrise's true killer had a more prominent target in mind, probably Sir Gawaine, and will likely try again. Playing the role of detective, he forms an uneasy partnership with Mordred in a quest to locate the vanished Lancelot and unmask the real culprit.
Kay investigates the actions and motivations of a number of the characters in the Arthurian stories, including a sympathetic Mordred, a somewhat lessened Lancelot, Morgan Le Fay, Sir Bors, Nimue, and others.
In the end, Guenevere is cleared by Lancelot riding in to save the day, but the mystery is solved separately, and justice of a sort prevails.