Australian Arnold Rosen is celebrating his 70th birthday with his family aboard a cruise ship in the Mediterranean. By coincidence, on the same cruise ship, the American Gus Smith and his family are also celebrating his 70th. The two men meet. They are identical in appearance but clearly not related. Arnold is the son of Jewish Holocaust survivors, while Gus' parents were German - indeed, his father was a rabid anti-Semite. It seems they are doppelgangers, or 'doubles'. However, it may be more complicated than that. Not only were the two men born on the same day, it turns out that they were both born in the same small town, in post-war Germany. The two men's backgrounds are so different that despite their similarities, there is surely no possibility that they could be related... What will they uncover when they travel to Waiblingen, the small town in Germany where they were both born, to seek the truth?