Frank and Joe want to be just like their father, the famous Fenton Hardy, an esteemed detective. The boys live in Bayport with their parents and attend the local high school. Frank is 16 and Joe is 15 in the 1927 edition, while Frank is 18 and Joe is 17 in the later edition. Three miles from the Atlantic on Barmet Bay, Bayport has a population of about 50,000.
On the narrow, curvy road between Bayport and Willowville, a speeding car passes Frank and Joe Hardy, who are on their motorcycles. Out for a Saturday morning ride and an errand for their father, the boys are angry with the reckless driver. After venting their frustration, they continue to Willowville to deliver their father’s package. They decide to stop at their friend Chet Morton’s before returning home. On the way back, they investigate a car crash, but don’t see anyone.
At Chet’s house Frank and Joe learn that Chet’s car has been stolen. The friends talk about chasing the thief and going to the police but decide to first go back to the car crash site. They think the driver of the crashed vehicle and the car thief might be the same person.
Frank, Joe and Chet pursue their hunch for a while before heading back to Bayport and the police station. They find out from the police that someone just tried to rob the steamboat ticket office. After talking with the police, Frank and Joe go home and talk to their dad about the events of the day.
Because of differing eyewitness accounts at the police station, the brothers think there must have been two people involved. Mr. Hardy suggests it might have been the same person in all three incidents.
In the 1927 edition: Mr. Hardy gives the boys some tips for good observation skills. No new clues surface over the next week. Without his car, Chet walks and get rides with others to get to places that week. On Friday afternoon, Frank and Joe invite Chet to go swimming and fishing on Saturday with their group of friends. As the friends finish talking, Chet sees a farmer coming toward them that he can get a ride with. Saturday morning, the friends meet at the crossroads near the river.
In the 1987 edition: Mr. Hardy introduces the idea of the criminal wearing a wig and suggests that the boys go to the wig shops in Bayport. The boys start with shop owner, Mr. Schwartz. The Hardy family attends church the next day (Sunday). On Sunday while the Hardy boys are over for dinner, Chet receives a threatening phone call from a man about his car. An intruder is spotted on the farm. The intruder gets away, but the boys find a grey wig on the floor of the barn and discover a spare tire is missing. Later that night, Chet receives a call offering information about Chet’s car for a fee. The boys decide to pay the fee, and a man takes them to a body shop that has a car the same make and model as Chet’s, but the car is not Chet’s car. On Monday, Frank and Joe visit more local wig shops after school.
Frank and Joe decide to put together a search party for the next day. On Tuesday morning, they call their friends and the group meets at the Hardy home for a planning meeting. Chet’s father drives him to the meeting. Mr. Hardy volunteers to help in the search.
In both editions: Chet’s car is found undamaged and parked in a clearing among the bushes. Back in Bayport, the Hardy boys find out about a house robbery. Jewels and securities were taken from the library safe in the Applegate family’s Tower Mansion.
Hurd Applegate asks their dad to take the case. Frank and Joe are invited to go with their dad to Applegate's mansion, where Hurd talks with the mansion caretaker. The caretaker, Henry Robinson, is the father of Frank and Joe’s friend Perry (known as "Slim"). Mr. Robinson knows the combination to the safe and had recently made a deposit to his bank account. Hurd calls the police and asks them to arrest Mr. Robinson. They do.
In the 1927 edition: On Sunday morning, the Hardy boys return to the crash scene to investigate again, and this time they discover a piece of a red wig.
In the 1987 edition: After school, Frank and Joe search Chet’s car. They find a red wig in the trunk of Chet’s car.
In both editions: The brothers take the clue back to their dad, and he agrees to talk to Hurd and the police. Mr. Robinson is released, but the case remains unsolved.
After Frank and Joe see a reward poster in the city post office, they ask their father for a list of the stolen items. Mr. Hardy gives his sons a list of the stolen property and tells them he has distributed a list of the jewels stolen to pawnshops, jewelry stores and police departments. The boys’ father is encouraging yet also realistic. He also reminds them that they are friendly competitors on this case.
Frank and Joe decide to go back to where they found Chet’s car and investigate again. There they find a coat and hat in the bushes (and the wig in the 1927 edition; in the 1987 edition, the boys find two strands of red hair in the hat). Not sure what to do next, the boys show their clues to their dad.
The Hardy boys and Mr. Hardy decide to help each other. Mr. Hardy goes to New York in the morning to follow up on clues. Frank and Joe stay in Bayport. After several days in New York, Mr. Hardy finds a clue that connects the stolen wig, car theft and attempted steamboat ticket office robbery to a well-known criminal named John “Red” Jackley. Before Mr. Hardy can find out more about the mansion robbery though, Red Jackley crashes while trying to escape from the police and is hospitalized. Mr. Hardy returns to Bayport to wait to hear from the doctor. While Mr. Hardy was in New York, Frank and Joe find out their friend Slim has to drop out of school and go to work to support his family.
In the 1987 edition: Mr. Hardy asks his sons to come to New York to help him on the case. Frank and Joe help him follow up with the owner of the wig and do research. Then Mr. Hardy sends them back to Bayport. Mrs. Hardy bakes a ham and cake for the Robinsons and asks Frank to take it to them after school. The day is Monday. Seeing Callie between classes that day, Frank invites Callie to go with him. She agrees.
In the 1927 edition: One day after school Frank and Joe’s friend Callie Shaw invites Frank to go with her to visit the Robinsons. He agrees.
In both editions: When Mr. Hardy returns to Bayport, he learns the local police know about Red Jackley. So when the hospital doctor calls Mr. Hardy to tell him that there is not much time left for Red Jackley to live, Mr. Hardy wants to get there before the police do. Frank, Joe and a group of their high school friends plan a diversion for the Bayport police to delay their departure to New York. The diversion works.
Red Jackley confesses before he dies, but is only able to verbalize a few words. Frank and Joe follow up on that clue and search the Applegate towers, but don’t find the stolen property. Mr. Hardy goes back to help them search. Mr. Applegate helps, too. Still nothing is found. Mr. Robinson is arrested again.
After a week, there are no additional clues — until Frank and Joe take a motorcycle ride and end up by an old railroad yard. The boys remember that their dad said Red Jackley worked for the railroad. They see two water towers and search them. Frank and Joe find the treasure and solve the case.
Frank and Joe take the treasure back to Bayport to show their dad. Mr. Hardy contacts Hurd, and they arrange a meeting at the Applegate mansion with the police, asking them to bring Mr. Robinson with them. Hurd and Mr. Hardy explain the new evidence that will clear Mr. Robinson. They show the chief the recovered treasure and tell him where the boys discovered it. Hurd apologizes to Mr. Robinson and offers to employ him again. Frank and Joe get a reward.
In the 1927 edition: At a celebration hosted by the Applegates for the boys and their friends, Slim is finally able to tell his friends how his dad got the money he deposited in the bank account — a person who owed Henry Robinson money paid him back. Hurd tells the group that they are welcome anytime.
In the 1987 edition: The money Mr. Robinson deposits in the bank came from Mrs. Applegate as a loan to help him through tough times. The book closes with Hardys leaving the meeting at the mansion and talking about the reward money. The boys want to put some in the bank and use some to build their own crime lab.