After the dramatic events of their mission to St Petersburg, Sophie and Lil know the truth - there's a double-agent at work at the Secret Service Bureau. Whilst Lil pursues a dangerous line of enquiry in London, Sophie must set out on a new mission to Venice, following a twisted trail to discover long-buried secrets. But there are villains lurking amongst the city's piazzas, canals and crumbling palaces, and in the shadows an old enemy lies in wait...