The story of Merope, the missing sister, is waiting to be told . . .

Following Georg Hoffman's revelation that he may have found The Missing Sister, Maia and Ally discover that all they have to go on is an address of a vineyard in New Zealand, plus a drawing of an unusual star-shaped emerald ring. Deciding that CeCe, who lives in Australia, is the closest, they send her to investigate with her partner, Chrissie. So begins a race against time to identify the Missing Sister, so she can join her sisters on The Titan to lay a wreath at a spot on the Aegean Sea where Ally last saw Pa Salt's boat.

It's a race that takes them across the globe, as Mary McDougal, the woman who has the emerald ring that can confirm for certain if her daughter, Mary-Kate, is the Missing Sister, has embarked on a world tour after the death of her husband.

As each sister takes their turn to trace her in New Zealand, Canada, England, France and Ireland, the elusive Mary manages to slip through their fingers,

and it seems that she does not want to be found...