Luna would do anything for her best friend Milly, but why did she agree to spend a week roughing it on an island?

Alex, a straight-A student and competitive swimmer, loves the idea of camping on a beach with his friends Otto and Jasper, until they stumble on a fortune in gold bars and are pursued by smugglers with guns.

When the boys don't arrive at the agreed meeting place, and the adults are all MIA, it's up to Luna and Milly to find their friends before it's too late. But is the mysterious local girl who claims she can help them a friend or a foe?