Frank and Joe Hardy, sons of a renowned detective, embark on their own amateur sleuthing careers. Their first case involves the theft of their friend Chet Morton's yellow roadster, which they link to a reckless red-haired driver previously involved in a crash and a steamboat office holdup attempt. Concurrently, their father, Fenton Hardy, is hired to investigate a major robbery at Tower Mansion, where the caretaker, Henry Robinson, is falsely accused and arrested. The Hardy boys uncover crucial evidence—a hidden red wig, hat, and coat—proving the thief used a disguise, leading to Mr. Robinson's initial release. Fenton Hardy pursues the professional crook, Red Jackley, to New York. Jackley, fatally injured during an escape, confesses to the robbery and claims to have hidden the loot in the mansion's 'old tower.' However, the treasure remains elusive, resulting in Mr. Robinson's re-arrest. The boys orchestrate a diversion to aid their father's investigation and ultimately locate the stolen Applegate treasure (jewelry and bonds) hidden in an old water tower. Their discovery clears Henry Robinson's name, earns them a reward, and triumphantly concludes the 'Tower Mansion mystery,' setting the stage for their future adventures.
