"The High Miles Mystery," winds up with a race that could spell life or death for the girl detective! Hinkley is able to build a prototype for a new high-efficiency car based on a recently recovered engine. After seeing Nancy drive a tank and stop a speeding train, he wants her (and mechanic Bess) to drive the prototype in a government-sponsored race. The car with the best fuel efficiency wins a government development contract, money Hinkley and Credo would need to bring the car to production. But someone is determined that Nancy and Bess lose-- at any cost!