It's 1961 and the Queen is planning her state visit to Italy aboard Britannia. But before she goes, an unreliable witness claims to have seen a brutal murder from the royal train.

Did it really happen, and could the victim be a missing friend of Princess Margaret's new husband, Tony Armstrong Jones? The Queen and her assistant private secretary, Joan McGraw, get to work on their second joint investigation, little imagining that this time it will take them all the way to Venice in a tale of spies, lies and Cold War skulduggery.