As a visiting forensic pathologist to Glasgow, Scotland in 1977, Dr. Daniel Kahn is called upon to examine a skeleton with a signet ring, buried in a shallow grave. He and his partner, DC Ian Marks, soon discover that the ring holds the insignia of the Mary Queen of Scots Brigade, a clandestine Scottish revolutionary group that intends to murder Queen Elisabeth II and replace her with a newly discovered descendant of Mary Queen of Scots. The pair of sleuths, working with MI5, discover that these insugents are being aided by the PLO and the Red Army Faction from West Germany. Charlotte Gordon, Daniel's cousin, and a Mossad agent, arrives to kills these foreign agents aiding the Scottish revolutionaries. It's nearly too late to stop the Mary Queen of Scots Brigade when the Queen comes to Dumbarton Castle to celebrate its restoration after the Nazis had bombed the town in 1941.