When librarian Laura Poole sees something strange in the darkness through the window of a London Underground train, she never imagines that she’ll soon be stumbling across a hidden people living below the city in the network of tunnels known as the Riverways. Taken into their confidence she is drawn into their world and there she meets Joseph, the man who for years has been their link with the outside.
But something is happening to the water in London and danger is coming from the sea. An old rivalry is reawakened and the network of secrets is no longer strong enough to protect them. A conflict is approaching that threatens London and Laura becomes a pawn in the struggle. Rescued then abandoned by Joseph, she can’t just stand by doing nothing. Librarians are made of sterner stuff than that, and she sets of to save her city.