When two young cousins exchange identities and places, one going to art camp, the other to a tiny village in Canada, the summer vacation becomes a mystery within a mystery. An old mill breathes unaccustomed life as the family paintings long stored there begin to disappear. Mary, posing as niece Sherin, goes to investigate the light in the mill and finds a number of exciting clues. Mary tries to carry on the hoax but fails to deceive Sherin's childhood friend, and they both have good fun in their respective impersonations. Girls will like the twittery plot, the art and romance, family feuds, and cats -- all of which evolve gently toward the happy ending.