Frank and Joe Hardy introduce their new mystery.

Iola's purse is stolen, and Frank and Joe track the suspected thief onto Silas Applegate's property. They spot Silas' plumber and hired hand before Silas comes out and orders them off the property.

Rumor of hidden treasure belonging to old Mr. Applegate spurs a summer-long investigation by the Hardy Boys

Joe finds Perry hiding in some bushes. Perry gives Joe a gold doubloon and asks him to hold it, then turns himself in.

The boys find a stash of stolen items from the community in Perry's living quarters. Though he is innocent of the thefts, Perry runs away to avoid capture by the police, when Joe stumbles upon his hiding place and learns the truth.

Rumor of hidden treasure belonging to Old man Applegate, spurs a summer long investigation by the Hardy Boys

The Boys go looking for treasure, and end up inside Applegate's house.

Applegate relates to Frank and Joe how he got the gold and how it was stolen.

Someone digs up Applegate's garden to get Frank and Joe in trouble.

While Frank and Joe wait for their father to get home, Iola goes to the Applegate house to retrieve the pick and shovel.

Hiding from Applegate in the tower, Iola finds Jackley unconscious and tied up. Her screams brings Frank, Joe, and Fenton running.

Fenton discovers Boles, the former gardener, hiding in the tower. The Hardys capture him.

Frank, Joe, and Fenton search Boles apartment while the prisoner taunts them.

Joe traps Boles into a partial confession.

The Boys find a clue they think will solve the mystery of the Applegate treasure.

The authorities look for the treasure in the tower.

The Hardy bunch and police tear apart Applegate's tower structure looking for the stolen treasure.

Frank and Joe spend a day following Boles after he is released from jail - and get another idea of where the treasure is hidden.

The Boys follow Boles back to his apartment but what they see from there leads them to deduce the location of the hidden treasure and bring the case to a conclusion.