Mog and Nick travel to Calcutta, in India, the country of their birth and the supposed location of the mysterious Winter diamond. Their family has owned the diamond for generations, and when they learn that a ruthless sea-captain is intending to seek it out and make his fortune, they determine to beat him to it and claim what is rightfully theirs. But when they arrive in Calcutta, they soon learn that the diamond also has far more sinister powers, and as they pursue their quest, the shadowy figure of Damyata is never far away...