In misty Shelwich, magic rises and falls with the tides, everyone is born with a magical gift, and people have started to . . . disappear.

Rumor has it they are snatched by monsters, but Ista Flit doesn't want to believe it. Not when her own father is missing. As she scours the city, searching for clues, a notorious thief finds her. Ista has a rare talent: she's a face-changer, able to take on the appearance of anyone she's seen, and Alexo could use a girl with her skill. Ista could use Alexo's help in her search--if only she could trust him.

As more people disappear from the city, Ista finds unexpected allies in Nat and Ruby, each searching for their own missing loved ones. These three determined kids will need to use all the Tidemagic they can summon, infiltrate the city's highest elite circles, and dive into its lowest caverns, in order to find how thieves, monsters, and the missing are all connected.