Grandfather's Aunt Sophie has died, leaving him the house in Brockton, Michigan, that he used to visit every summer. The Aldens go to clean it up so that it can be sold.
Looking through the old house is fun. Especially when they find an old diary from 1900. The diary mentions that a diamond and pearl ring is hidden in the house. The only way to find it is to solve a riddle written in the diary. But after solving the riddle, the Boxcar Children discover the ring isn't in the house anymore; someone sold it to a pawnshop. Who could have sold the ring?