Plot summary (original edition)

The boys (Frank, Joe, & Chet) go see a movie called "A Figure in Hiding" at the local Rialto Theater. After the film is over, the Rialto is robbed with the thief getting away with $900. Then the boys go to the Bayport Hotel and meet up with their father, Fenton Hardy. They spy on a pair of men (Rip Sinder and Spotty Lemuel) in the room next door who are part of a cruel gang called the "Eye Syndicate", ran by a bogus doctor named Doc Grafton. While Sinder and Lemuel are doing Grafton's business, Sinder's adopted daughter barges in and begs him to come home. Spotty, trying to get her to leave, reveals to her that she was adopted, something that Sinder had kept secret from her. This works and Virginia runs away in a fit of despair and sorrow. Not long after that, the police mistakenly arrest their friend Chet for the robbery. It was really Nick Cordoza, of whom the boys found in his car unconscious. Chet runs away to Boston instead of going back home due to a note supposedly from the Hardys to go there. Later, their father devises a plan to trap Grafton by telling one of his clients (Henry Zatta) to pay him with marked bills. However, the plan falls apart after the boys and Grafton get into a fight, causing the quack to escape without the $500 in marked money. Eventually, the boys meet Virginia at an auto garage, where they chase her to Grafton's hideout. Grafton and his mute assistant Zeb drug them and attempt to bury them alive but the drugs wears off too fast and a fight ensues. The boys are able to make it out of his cellar and the police show up to arrest Grafton and his lackeys. After the case is solved, it is revealed that Dr. Grafton's name was revealed to be Paul Zane, Mr. Zatta's son.

Plot summary (revised edition)

While leaving the baseball field the Hardy boys are approached by a blind peddler with a warning for their father Fenton Hardy. Then the boys take their boat, the Sleuth out on Barmet Bay for a ride and to watch the testing of a new hydrofoil boat named the Sea Spook. While admiring the new boat they almost have a collision with it, allowing them a reason to board the boat where they find a glass eye, presumably belonging to Mr. Lambert who was test driving the boat. When the boys try to track down Mr. Lambert he goes missing and Bill Braxton, the owner of the Sea Spook is attacked at his boathouse and the Spook is searched. Meanwhile Fenton Hardy examines the warning from Henry Zatta, the blind peddler, and declares that it must be warning them to keep away from the Goggler Gang. They then realize that Mr. Lambert is actually a hoodlum named Spotty Lemuel. Their discussion is interrupted by an old man at the door named Zachary Mudge who is staying at Doc Grafton’s Health Farm, a luxurious resort overlooking Barmet Bay. Mr. Mudge tells them that he would like to purchase the Sea Spook. The boys decide to go to a movie at the Bijou but as they arrive they see a man wearing the disguise of the Goggler Gang running away with the cashbox. Later, the boys travel up the river in the ‘’sleuth’’ to speak to Mrs. Lunberry, the owner of a Jeweled Silva which was recently stolen, on behalf of their father who is going to take on the case. Upon returning they are summoned to Doc Grafton’s Health Farm by their chum Chet Morton, who has taken summer employment there. During the course of their travels they keep seeing suspicious vehicles, specifically brand new Torpedo sedans. This clue leads them to Izmir Motors, owned by Malcolm Izmir. The boys try to search Mr. Izmir’s house but are attacked by his guard dogs. Upon returning home they find their house has been burglarized and their fathers safe blown open. They learn from Chief Collig that the Bijou holdup man was actually Nick Cordoza, and that he had broken into Mr. Izmir’s house, but Mr. Izmir had refused to press charges. The boys also learn from Mr. Mudge that the Izmir syndicate has gone broke. They also suspect that Doc. Grafton’s resort is actually a hideout for crooks who are getting plastic surgery to alter their looks, based on Chet’s report that he saw a mummy in an off-limits building on the Health Farm and their guess that Mr. Izmir is the head of the Goggler Gang because his name can be shortened to Mal I, which may mean ‘evil eye’. Fenton Hardy goes undercover at the resort while Chet and the Hardy boys investigate by sneaking on to the Health Farm. They all end up captured by Spotty Lemuel and Rip Sinder. They are rescued when Chet calls the police, who arrest the villains and several other crooks that were staying at the Health Farm.

Original edition (1937)

• Ghostwriter: Leslie McFarlane
• Summary: The boys foil the "Eye Syndicate", a gang run by the phony eye surgeon Doc Grafton.

Revised edition (1965)

• Revision type: Completely rewritten; some themes and characters from the original are adapted into the completely different plot, which involves a theft ring and a facility offering plastic surgery to criminals.
• Ghostwriter: James Duncan Lawrence