Alastair Maclean, a Scotsman who has been living in France with the exiled Stuarts, comes to England to join the Scottish army as it advances towards London. But on the way he discovers that agents (Sir John Norreys and Nicholas Kyd) supposedly helping the Jacobite cause by encouraging various nobles to commit to the cause, are actually in English pay, and are passing on to the English government the letters from these nobles to Charles Stuart promising...