"Deryni Checkmate" takes places in March 1121, four months after the coronation of fourteen-year-old King Kelson Haldane. The novel opens with the rabidly anti-Deryni leader of the Holy Church, Archbishop Edmund Loris, signing a letter that demands that the Deryni Duke of Corwyn, Alaric Morgan, recant his magical powers and submit to a life of penance. If he fails to do so, Loris threatens to excommunicate Morgan and place his entire duchy under interdict. Additionally, Morgan's cousin, Monsignor Duncan McLain, is suspended and summoned to an ecclesiastical trial to answer for his part in the events surrounding Kelson's coronation. After being warned of the threat, Kelson sends Duncan to warn Morgan of the upcoming trouble, fearing that Duncan's hidden Deryni heritage may be revealed by a trial.