Returning from business abroad, Karl Frank, a young attorney and investigator, is captivated by a beautiful young lady, the Countess Margit von Kral, who is travelling with her brother, Count Kuno von Kral. Frank becomes well acquainted with both, and they ask him to visit them at their hotel one of the next few days. Frank accepts and then hurries to file a travel report with the investigative police. Right away, he gets a new assignment: the police have learned that an attempt will be made to burgle Schwartz’s jewellery store that night and Frank is charged with catching the thief.
Later, he and the jeweller catch the thief red-handed. But the thief is crafty and in the blink of an eye he has hogtied both Frank and the jeweller. Following this fiasco, Frank accepts an invitation to spend a few days at the von Krals’ estate. His fondness for Margit grows and he proposes to her, incurring the wrath of her brother who forbids him from ever setting foot in the manor again. Unable to live without Margit, Frank sneaks in behind the castle fence, but he is jumped by a dog – and the count.
To Frank’s astonishment, the count is none other than the burglar who outfoxed him at the jewellery store. He is taken to a cellar piled high with the count’s loot. The mysterious aristocratic criminal lets Frank in on his final master plan to steal the crown jewels. All he needs is a partner he can trust: if Frank agrees to help him out, he will allow him to marry Margit. When a watchman discovers them in mid-caper, Frank gets away by showing his police badge, while Count von Kral makes off on his own. Back at the manor, he summons his sister and together they escape to a new country and new exploits. Meanwhile, the police conclude that an explosion at the manor means that von Kral chose death above capture.