Left with only a single clue that leads her to Lisbon, Aventurine Morrow must attempt to follow it to determine what has happened to her brother-in-law. Was he lost at sea while soloing the Atlantic, as everyone has assumed over the past two years? Or is there something more nefarious happening?

Observing an uneasy truce with her sister Micheline and nephew Paul, Avi searches blindly for some-any-information to solve this heartbreaking and terrifying puzzle. She finds a surprising source of aid from sometime-nemesis, DCI Dominic Burroughs; at the same time, she's seemingly thwarted at every turn by her half-sister Nicola, whose aims are unclear, but increasingly cruel.

When things heat up in Portugal, Aventurine and her friends are forced to flee, back to the seaside village of Whitby in the north of England, where everything comes to a head, and they discover that all they thought they knew is an illusion.