Frank and Joe solve the mystery of the secret locked room in the spooky Dalrymple Mansion.

A banker who has been receiving threatening notes enlists the help of the Hardy boys. Before long, the young sleuths find themselves entangled in the investigation of a notorious band of thieves.

To their friends’ envy, Biff and Chet plan to take a boating vacation up the coast. The joy of sending them off soon turns to anxiety as neither of them make contact with home for several days. Convinced that something’s happened to them, the Hardy boys and their friends go on a search filled with adventure and peril in hopes of retrieving their missing chums. This is the fourth book of the Hardy boys series, first published in 1928. While the author is credited to be Franklin W. Dixon, in reality, Leslie MacFarlane and Edward Stratemeyer are primarily responsible for the early volumes, including this one. This Standard Ebooks edition is based on the original 1928 text. This book is part of the Standard Ebooks project, which produces free public domain ebooks.

"What Happened At Midnight" is an exciting mystery novel by Franklin W. Dixon. The story revolves around the adventures of the Hardy Boys, Frank and Joe, who are skilled amateur detectives always eager to take on challenging cases. The plot begins with the Hardy Boys receiving a mysterious message requesting their assistance in solving a perplexing and dangerous case. Intrigued, they set out to unravel the enigma and uncover the truth behind the strange events that occurred at midnight. As the brothers delve deeper into the investigation, they encounter a series of thrilling escapades and encounters with suspicious characters. From hidden passages to cryptic clues, the Hardy Boys use their intelligence, bravery, and quick thinking to navigate the twists and turns of the case. As the story unfolds, the brothers find themselves entangled in a complex web of deception and danger. They must rely on their detective skills and work together to decipher codes, follow leads, and uncover hidden secrets. Throughout their quest for the truth, the Hardy Boys face various challenges, including narrow escapes, daring rescues, and unexpected revelations. With every clue they uncover, the mystery deepens, and the stakes become higher. In their relentless pursuit of justice, Frank and Joe race against time to solve the puzzle before the clock strikes midnight once again. The climax of the story is filled with suspense and excitement, as the Hardy Boys finally uncover the truth behind the mysterious events that took place in the dead of night. "What Happened At Midnight" is a thrilling and engaging mystery that keeps readers on the edge of their seats from start to finish, showcasing the bravery and resourcefulness of the Hardy Boys as they solve yet another intriguing case.

A plot to steal a top-secret instrument vital to the United States space program poses a challenging case for Frank and Joe Hardy. After rescuing a South American stowaway who mutters a warning about “Footprints “ and then vanishes, Frank and Joe discover that documents belonging to their famous detective father are missing and that footprints linger under a window. The young detective’s search for the diabolical mastermind of the “Footprints” spy ring takes them on a flight to a group of islands off the coast of South America.

A clock in a vacant house provides a clue to the whereabouts of a gang of harbor thieves

An archaeological expedition in the West turns into a desperate attempt to capture robbers and an escaped convict.