One of the key elements of the plot is a stuffed whale. That is a bit absurd, but it gets even worse when the whale is unearthed by Tony Prito's construction company. No mention is made of the condition of the whale, but it is hard to believe that it would be intact after having been buried for some time. The story is that it was part of a carnival that went bankrupt and they just buried the whale where it was.
The whale contains a carved ivory statute and there is a gang trying to recover it. They all have a tattoo of a whale, so that is the origin of the title. There is also a modern carnival, some heroics by Chet Morton in rescuing the kidnapped Hardy's and significant contributions by Fenton Hardy. The best part of the book stars Chet Morton. He consumes a record number of ice cream sundaes, which was made funny by him having a set of admirers. When Chet is preparing to consume the last sundae, he jogs around the block, followed by two admiring boys.
The remainder of the plot of the book does not remain viable. The names of the people are somewhat artificial, for example Fenton Hardy poses as a seaman and takes the name Spike Marlin. This is a flip of marlinspike, a tool used by sailors. Knocker Felsen is a tough guy who takes on the Hardy's and gets beat up. Therefore, while there are high points to the book, they are not enough to make the book better than weak.