The summer was supposed to be over, and the danger was supposed to be behind them, until a gold coin turned up on Gemma's breakfast plate. Weeks after escaping Captain Gordon's smugglers, Gemma, Nicholas and Mot are back in the caves beneath Hartley, chasing a trail of hidden keys, tide-locked doors, and a secret that Trunky Morgan buried two centuries ago. Nicholas is finally learning to master the impossible gift that once nearly got them all killed. Mot is learning he doesn't have to be afraid anymore. And Gemma is certain the real treasure is still down there, waiting for someone patient enough to find it. But someone the town thought was gone for good hasn't given up. Victoria Gordon is back, colder and far more dangerous than her father ever was, and she wants what the caves are hiding just as badly as the three friends do. The tide only opens the door once a year, and this time, not everyone who goes in is coming back out unchanged.