The Message in the Hollow Oak is the twelfth volume in the Nancy Drew Mystery Stories series.
Nancy Drew finds out that she has won a rather unusual prize in a contest, a piece of land in Canada. She takes a trip, her first outside of the United States to see what her new property looks like.
As she is traveling by train to Canada, she meets an authoress, named Ann Chapelle, who is also traveling to Canada. Suddenly, the train crashes, and everything is thrown into confusion. Nancy and her two friends, Bess Marvin and George Fayne, are uninjured, but Chapelle is taken to a nearby hospital, gravely injured. When Nancy and her friends find her, Miss Chapelle tells Nancy the reason she was going to Canada, and asks a favor of her - to give a message to Miss Chapelle's grandfather, and to a lost love whom she hasn't seen since she ran away from home some years ago.
Along with this request, Nancy also has another problem: Two men have heard that there might be gold on Nancy's land, and are determined to get there first.
New York City detectives can't find a clue to a missionary's fortune, which is hidden in a hollow oak tree. Nancy goes to a burial site in Illinois that is connected to the mystery.