Cousins Marisol and Jake are at the packing up of their grandmother's house before the sale when they decide to boat out to a lighthouse, following a map they found. They lose the boat, they find a key, they use the key, and they find themselves -- in another world.
A world of lost things.
Christopher Creaturo explains that he and his fellow Curators could get them out, but there's a lot of red tape, and if they steal a ledger for him, he can make sure it's done irregularly but in time to get them out.
Complications ensue. Losing memories, getting another -- irregular assignment, learning more (and then more) about Christopher, a jungle, several underwater cities and a woman who dances in one, what Jake and Marisol think about loss and finding, a very jumbled royal court, and more.