Thus far each of the first three books in the Hardy Boys series has increased the drama and complexity of the stories. The fourth book raises the ante once again by having at least four, apparently different, mysteries.
At the beginning of the book the boys take their new boat, the Sleuth, out on the bay. While they are cruising on the bay another boat nearly rams them. They would have chased the boat but the steering on the Sleuth broke, and the boys ended up going around in circles. It turns out that the boat that nearly rammed them had a purpose for doing so, that we do not discover until the end of the book.
Soon after, the boys prepare to go to Callie Shaw's costume party. Frank likes Callie romantically and she is a recurring character in the Hardy Boys stories. They encounter another mystery as it appears that men in Mr. French's costume shop appear to be threatening Mr. French.
Returning home the boys frighten their Aunt Gertrude with their costumes. Aunt Gertrude is yet another recurring character in the series. Soon the boys are off to the costume party on their motor cycles. On the way they realize that the bank is being robbed. They follow the criminals until they lose them at the docks, where they hop into a boat and escape into the fog.
After notifying the Coast Guard, the boys gain permission from Chief Collig to search for the criminals in the Sleuth, but the boys discover the Sleuth has been stolen! The boys search for the bank robbers in Tony Prito's boat, the Napoli, but are unable to find them in the thickening fog. The boys return home, explain to their father everything they saw at the bank and during the chase, and then head out to the costume party.
The next day the boys awaken to learn that Chet Morton and Biff Hooper never made it home from the party. The boys not only have to learn who stole the Sleuth, but where their missing friends went, and who robbed the bank. As the story develops the boys learn that expensive radios that may have been stolen are turning up. Lastly, a hermit on a tiny island with a shotgun threatens the boys.
The Hardy Boys face many mysteries and yet the author managed to clearly explain how the mysteries did or did not relate to each other. Each of the first four Hardy Boys mysteries were quick, enjoyable reads, and I would recommend them for children in the age range of about eight through whatever age the series holds their interest. Though the Hardy Boys series is written in a relatively archaic fashion, as reading material for an increasingly younger audience they are excellent. The stories were once recommended for children ages 10 to 14. As children are exposed to more violence and seem to require greater levels of stimulation, the recommended age range has move to 9 to 12. I think any child capable of reading some of the challenging words in these books will enjoy them, regardless of how tame most of the action may be. Once a child has reached age 12 or so the stories may be of less interest, but given the combination of mystery and action, these books remain good safe choices for parents who want to know what their children are reading.