It's 1926 and life is good for Kit Hardwicke and Robbie Ferguson and most of their friends. Kit is learning that he can travel for fun if he wants to, not just to solve problems of national security. They have a summer holiday planned, cruising around the Aegean. In part this is because Robbie wants to revisit a small island he briefly visited just before the war, which has haunted him ever since, but mostly he just wants to show Greece off to Kit and have a good time in the sun. Laura and Dixon are coming with them. Their only real worry is their friend Mack, who has suffered some setbacks in his professional and personal life and has sunk into a deep depression. He is coming along too and they hope the trip will cheer him up. It does, but only after much mystery, a lot of fog and misunderstanding, and an odd pair of twins who seem to have become their responsibility rather accidentally.