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, and fast-otherwise, Amy will have to sell the retreat. As the riddle starts to unfold, strange things start to happen around the lake. Something is moving around in the water late at night, and someone doesn't want the Aldens helping Amy. Will the Boxcar Children crack the riddle before it's too late?