The students are Starhurst are excited about Mrs. Crandall's plans to purchase a piece of property at Moon Lake for a girls' camp. Five girls, including Louise and Jean Dana, earn the privilege of accompanying Mrs. Crandall to the proposed site on a weekend outing. Much to the Danas' dismay, Lettie Briggs makes the trip as well since Mrs. Crandall has promised her parents to look after Lettie while they are away.
The trip proves to be exciting, because the house located on the proposed camp property appears to be haunted. At any rate, someone else is staying in the house inside the locked attic room and only comes out at night when the Starhurst group is asleep.
With difficulty, the girls convince Mrs. Crandall to extend their stay so that the mystery can be solved. If the girls can prove that the proposed camp property is not dangerous, Mrs. Crandall will purchase the property for the school.