The book takes place several months after Deryni Rising, and details the consequences of the events that surrounded the coronation of young 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.

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.

In addition to the ecclesiastical threat, Morgan's duchy is being ravaged by an anti-Deryni fanatic named Warin de Grey, and the neighboring kingdom of Torenth is preparing to launch an invasion of Gwynedd.

Fearing that an internal Gwyneddan conflict will weaken the kingdom prior to fighting an external enemy, Morgan and Duncan eventually decide to travel to the city of Dhassa and personally appeal to the Curia of Bishops.

The other finalists were Ursula K. Le Guin's The Farthest Shore, Poul Anderson's The Dancer from Atlantis, Thomas Burnett Swann's Green Phoenix, and Roger Zelazny's The Guns of Avalon.

In 1973, Deryni Checkmate ranked 15th in an annual poll of Locus magazine readers, placing it between Jack Vance's The Brave, Free Men and Barry N. Malzberg's Beyond Apollo.