When Mary and her sister Betty visit the Ivy Bay lighthouse with their friend Henry, Mary stumbles upon a crude map drawn on the wall. Henry wonders if it might be connected to his ancestor David, who was accused of stealing a large sum of money and a precious family heirloom. Mary decides to investigate, but solving a crime committed almost a century ago isn’t easy, and a surprising number of people don’t want her to succeed. With so many obstacles blocking her way, can she discover what really happened with Henry’s ancestor? And where are the money and sapphire ring that went missing all those years ago? Even more important, can she find these treasures before anyone else does?