The Mystery of the Spiral Bridge

In the thirty-sixth book of the Hardy Boys series, the Hardy boys face a difficult challenge when a new invention, a machine that can make a man invisible, is stolen from their father's client. The brothers travel to Kentucky to investigate a series of strange occurrences at a recently constructed bridge, finding themselves drawn into a web of danger and intrigue that requires all their detective skills to solve. Frank and Joe Hardy are determined to bring to justice the vicious criminals who kidnapped their detective father while he was investigating sabotage of a road-building project in the Kentucky wilderness. After Mr. Hardy’s escape, the two young sleuths are spurred into action when he mumbles the word 'Felix' and they discover a dossier of a notorious ex-convict was stolen from his files. The trail leads them to New York City but ends abruptly in a cemetery. With the new clue of a spiral symbol, the boys head to Kentucky as part of a highway construction crew to track down the kidnappers and saboteurs.