The Happy Hollisters are excited about the upcoming autumn season, with plans for pumpkin decorating, bobbing for apples, and the Shoreham Halloween parade. They decide to host a party for their friends and Farmer Johnson offers the use of his barn for the festivities in exchange for the children's help selling pumpkins at his farm stand.

The mysteries begin almost right away, when the horn on Farmer Johnson's old Model T Ford begins to blow . . . when no one is in the car! Pete and Pam spy a girl in a red jacket running away from the barn, but she escapes into the woods before they can ask who she is. On the chase, they stumble on an ancient gravestone behind the barn--upon which is written a cryptic message about a hidden treasure. Then a stranger asks for the children's help finding a weathervane in the shape of a golden witch. Could all of these mysteries be related, and will they be solved before they interfere with the children's Halloween festivities?