Racing cars! Mystery! Adventure! These elements combine to give Frank and Joe Hardy one of the most dangerous and intriguing cases of their careers. It all starts when their father, Fenton Hardy, is engaged by an experimental race car and motor designer to investigate a series of mysterious accidents. Three of his drivers have crashed because the windshields of their cars were suddenly crazed, thus cutting off forward vision. Frank and Joe uncover one slim clue. Each of the drivers had seen a signpost marked DANGER shortly before the accident. The young detectives investigate, only to discover that the signposts have vanished. What happened to them? And what sinister purpose did they serve? 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.