Inspector Willoughby captures infamous and elusive master spy X13 after a lengthy chase aboard the Cloak and Dagger Express. Seward Willoughby, famed criminologist, is on his way to a foreign embassy to apprehend international spy X13. Standing in front of the embassy, as Willoughby approaches it, is X13 disguised. Belatedly, Willoughby recognizes the spy and uncovers his disguise. The spy jumps into a taxi cab and heads to the airport, with Willoughby in close pursuit. X13 gets aboard the plane, thinking that he has lost Willoughby, but he’s seated on the plane’s tail with a stewardess in attendance. To head off X13 at his destination, Willoughby, using an umbrella, parachutes to the ground and, seeing X13 headed for the railroad station, follows him. As he’s about to board the Cloak and Dagger Express, X13 is arrested by Willoughby, handcuffed and led away. A “weight” machine causes a card to fall out which directs Willoughby to his quarry. Catching the train on which X13 is a passenger, Willoughby finds him in Compartment A. A series of incidents frustrates his efforts to capture the spy. Eventually, X13 is apprehended in the mail car and put in a mailbag. When he finally opens the mailbag, Willoughby discovers that he has the wrong man, and he philosophically remarks, “Oh, well, you can’t win them all.”
