Summoned to Hanley Field on a mysterious assignment, Hop Harrigan and Tank Tinker boarded an early train and were on route when Hop received a telegram from Captain Clarke, acting commander of the field, to get off the train at Oakdale, where he would meet them. Previously, Hop had learned from reading a newspaper that a cargo plane had been mysteriously forced down the previous day, and the boys decided that was why they'd been called in...