The Mystery of Shark Island

Marooned on a remote outpost in the Greek archipelago, BJ, Graham, Paul and Terry think they are safe from the criminal syndicate they helped to expose.
It is not long, however, before a chance find of a thumb drive throws them into another mystery, and triggers a chain of events that puts their lives in mortal danger.
Who is the ever-present Cigarette-Smoking-Man, what data can possibly be on the thumb drive that is so important to desperate men, and who is sending mysterious signals from Shark Island?
From a cave supposedly inhabited by a Gorgon, to an old castle that really isn't, and a secret military base, their supposed getaway soon turns into conflict with a deadly ring of spies.