Forecast From Stonehenge

From the back of the book. The day you get back to London from an archaeology dig, your friend Twig has an urgent request. Can you meet a man named Alastair at the Stonehenge monument? Alastair claims to have knowledge of the fabled missing "heelstone" so you agree to go. When you find Alastair, he is in deep trouble and passes along something more dangerous than some archaeological gossip.