When Maddy and her brother CJ arrive at Loutraki Bay for their annual summer break, they are looking forward to long days waterskiing with their dad and most of all, swimming with Indigo, the local dolphin. But Indigo is no ordinary dolphin: he is able to speak to certain humans, and has been friends with Maddy ever since saving her from drowning when she was a toddler. And this time, he has an urgent message to tell her: the dolphins are under threat from a mysterious underwater noise that interferes with their sonar, causing them to lose their way at sea. Maddy and CJ soon discover that the source of the noise is the American submarines, which have come to Greece to prevent attacks on the NATO summit in Athens. When the US President and his daughter arrive in Loutraki, Maddy and CJ see their chance to alert him to the dolphins' plight; but persuading him to call off the sub- marines will take a very risky plan. . .