In Chapter One (Modern Pirates), the G-Men learn that "Brenda", notorious jewel thief, is heading for the United States, to steal the Belgravian crown jewels currently on exhibit. The gems are placed on a ship bound for Belgravia, the guard is murdered and the jewels stolen. Secret Agent X-9 trails Blackstone, one of the gang, who hides the gems in a safe deposit vault of a bank. He takes the bank receipt to an art shop, where Marker, an accomplice, conceals it between an oil painting and its frame. X-9 arrests Blackstone and pursues Marker, and leaps into the speedboat Marker is getting away in.