The Aldens are visiting Grandfather's friend, Amy, at the Ogopogo Retreat in Canada. Amy rents cabins to tourists who hope to see the mysterious underwater creature, the Ogopogo, swimming in the lake. Henry, Jesse, Violet, and Benny can't wait to try to spot the Ogopogo from their cabin. Amy tells the children about a riddle that an old friend left for her in one of the cabins that could lead to a small treasure. The children need to find the treasure,...