Plot summary (original edition)

The original story involves the theft of a famous racehorse, Topnotch. Intertwined are the mysterious and malevolent works of the villain Vilnoff. These two stories are intertwined and the Hardy boys are immersed as they solve the seemingly unlinked mysteries.

Plot Summary (revised edition)

This story begins with Frank and Joe Hardy driving home along Shore Road when they have an accident with a dragster. When they arrive home their father, Fenton Hardy, tells them about a new case he has taken on for Alden Automotive Research and Development Company, with which he would like the boys' help. Mr. Hardy explains that Mr. Alden believes someone is trying to steal the plans for a secret new engine he is designing, and that two of his racing cars which were equipped with new engine had strange accidents in which their windshields suddenly turned milky white, just after passing a road sign warning “DANGER”. While they are discussing the case someone fires a smoke grenade into their house with a warning attached, telling them to "drop the Alden case".

Frank and Joe start by working undercover at Alden’s shop, while their friend Chet Morton takes an interest in jet propelling his bicycle, which leads to humorous results. At the shop the boys meet Barto Sigor and they learn that he has a twin brother Vilno who recently quit Alden’s employment. The boys immediately suspect that the twins may have swapped spots so that Vilno can gain access to the plans for the new motor. The Hardy boys also meet Roger Alden, Mr. Alden’s son, who seems to have a bad attitude. He is the boys' second suspect.

Aunt Gertrude receives news that she has just inherited a farm for retired racing horses. She is horrified at the prospect of owning such a thing, but before she can sell it the Hardy boys go to see the place. When they return they find that Mr. Alden’s racehorse Topnotch has been stolen and is being held for ransom. The Hardy boys return to the farm where they meet with Fowler, the manager of the farm, who tells them to get lost. While there, Frank picks up a cartridge from Fowler’s rifle, which he later matches to the rifle casing used to shoot the smoke grenade into the Hardy home. It is later revealed that Fowler is actually Norman Dodson, and is responsible for the theft of Topnotch. Using their detective skills, the Hardy boys figure out where the horse is being kept. They go there, get captured, but manage to escape while riding Topnotch to freedom.

Returning to the Alden automotive case, they learn that Barto has fled the plant with Mr. Alden's experimental race car. The Hardy boys eventually figure out where Barto is hiding, but when they reach the mansion hideout, they are suddenly "frozen in their tracks" by a "powerful invisible force". Once captured they find their father has also been trapped, along with Mr. Alden and his son Roger. While they are being held hostage Vilno brags to the boys about his "sonic trap" which can trap objects inside using "hypersonic vibrations" and he explains how he used his "hypersonic generator" to glaze the windshields of the race cars. Once Vilno leaves Joe manages to escape from the cabin through an air vent, where Chet Morton arrives on his rocket propelled bike, causing the villains to crash their car between two trees and trapping them inside until the police can arrive to arrest them.

Original edition (1936)

• Ghostwriter: Leslie McFarlane
• Summary: The boys track down a stolen racehorse and save Bayport from destruction by the madman Vilnoff.

Revised edition (1968)

• Revision type: Completely rewritten; two elements (a stolen racehorse and an evil inventor) are retained, but the plot is otherwise completely different, involving the theft of top-secret racecar engine plans.
• Ghostwriter: Tom Mulvey