In one of the most dangerous and intriguing cases of their careers, Frank and Joe Hardy help their father investigate a series of mysterious car accidents. Each of the drivers had seen a signpost marked DANGER shortly before his accident. The young detectives investigate, only to discover that the signposts have vanished. The attempted theft of a secret experimental motor and the kidnapping of a famous race horse are part of this thrilling case, which proves to be as sinister as the signposts themselves.

The Hardy boys and their father solve a mystery involving inexplicable crashes of highspeed race cars being used to test an experimental turbine motor.

A million dollars’ worth of gold bullion has been stolen from the Wakefield Mint under strange circumstances. Mr. hardy is asked to investigate but before long his life is threatened, and he asks Frank and Joe to help him. The boys fly to Zurich, Switzerland, hoping to get information at the Swiss Gold Syndicate and to find the man who has stolen a valuable ancient gold figurine from a New York museum. Their search on both counts seems futile as they return to the United States where they uncover clues that lead them to an adventure in the Yucatán jungle. But the Hardy’s travels lead to nothing but new doubts and nagging suspicions. And now their lives are in danger.

There is not time for explanations when Mr. hardy telephones Frank and Joe form the West Coast and sends them flying off to England to help his old friend Professor Rowbotham. Their stay in East Anglia begins with a weird omen, as they witness the bizarre funeral of an old witchmaster. From then on, strange things happen. When the Hardys learn about the strange disappearance of Lord Craighead, the plot deepens. Danger follows them to Ireland as Frank and Joe survive a shipwreck and find a clue from a frightened white which that leads to the torture chamber of a black witches’ coven, all in search of the telltale Witchmaster’s Key.

Valuable electronic parts containing platinum are being stolen from shipments made by Stanwide Mining Equipment’s cargo planes, and Frank and Joe are called upon to assist their world-renowned detective father solve the baffling case. While posing as Stanwide employees, the boys look into the truth behind the chief pilot’s death at sea. The puzzling trail of clues leads the young sleuths to an uninhabited Caribbean island, but the final, and most startling, discovery is made in the boys’ home town of Bayport.

When eccentric Thaddeus McClintock invites Frank and Joe Hardy to accompany him on a sea voyage, the teenage investigators become entangled in a web of mystery. Who is trying to block the three from securing reservation on freighter ships that carry passengers? The determined efforts of Frank and Joe lead to a hazardous game of wits with a ring of slippery smugglers and to a dramatic confrontation on the high seas. Here is an exciting, action-filled mystery that will keep the reader on edge with suspense.

The boys once again have jumped into the thick of a complicated mystery. Samuel Blacksone and his brother Ruel Rand live on adjacent properties, and it appears that one of them is out to hurt the other because they are both attacked. Frank and Joe soon learn that treasure is rumored to be hidden between the two properties, and as they investigate, they learn that others are involved in the apparent plot between the two brothers. Spooky mansions, secret passages and rooms, mysterious characters, and a monster provide an enjoyable atmosphere to this Hardy Boys mystery.

When the Hardy Boys eagerly agree to assist their friend Jack Dodd and his father in locating a family treasure, the two young detectives are unaware of the baffling complications that will arise. Deadly road traps, aerial ambush, a spider man and a suspect who carries a menacing cane harass the young detectives as they seek the triple solution to this perplexing and exciting mystery.

Two unexplainable plane crashes near an airport on the East Coast plunge Frank and Joe Hardy into a bizarre case. From the moment Frank and Joe find a radioactive engine in an airplane junkyard, unexpected dangers strike like lightning. Despite the repeated attempts on their lives, the teenage detectives pursue their investigation, discovering two vital clues and others that provide the solution to one of the most baffling mysteries the boys and Mr. Hardy have ever encountered.