Mannering visits Scotland after the death of one of his associates in a car accident; things take a mysterious turn when he spots someone in a local pub wearing the antique ring Mannering had instructed his associate to buy.

Templeton-Green assigns Mannering to uncover an art smuggling operation which is seeing priceless treasures from the Vatican being couriered to London. An old flame of the Baron is also involved, and Mannering is as keen to rescue her as he is to solve the mystery.