The notorious criminal Barra receives a visitor in the cavern hideout whence he wages war on society. The visitor is Agent Frank, a man willing to take on any kind of shady commission. Occasionally, for a particularly daring heist, he seeks Barra’s assistance. What is his plan this time? A rival of the worldwide Osman Corporation has learned that Osman’s trusted man, engineer von Rosen, is on his way to the city where Frank operates. Von Rosen’s mission: to bring to the director of the local Osman branch the plans for a new combustion engine. Frank’s job is to get the plans, which are worth 40,000 francs to his client. Barra agrees to help out in return for half the fee and immediately sets to work.
He has heard that von Spangen, the director of the local Osman branch, is advertising for a lady’s companion. Mrs Barra, who often lends her husband an invaluable hand, is furnished with a set of stellar references and gets the position. Mrs Barra informs her husband when the valuable plans arrive. Barra tries to steal them the very same day, but he is found out and barely manages to escape. Concerned there will be another attempt to steal the plans, von Spangen sends for the famous detective Asbjørn Krag. Mrs Barra tells her husband and he smuggles in a gun to her and shows her how to use it. As the gentlemen sit in the parlour perusing the plans, she fires a shot in the hall. In the ensuing confusion, they forget all about the plans and it’s easy for Mrs Barra to snap them up and toss them out to her husband who is hiding in the garden.
Detective Krag soon becomes suspicious of the impostor lady’s companion and has her placed under closely guarded house arrest. But Mrs Barra is unconcerned: she knows her husband will never let her down. Sure enough, Barra has devised a clever plan. Disguised as an electrician, he snatches a sleeping infant and lowers it down on a rope to his assistant. A note he left at the cradle instructs von Spangen to give Barra safe passage to collect his wife. In return, the child will be returned. So it goes, but detective Krag has cleverly rigged a petroleum can underneath Barra’s parked car, and as Barra and his wife drive away, a petroleum trail clearly shows which way they went. As soon as von Spangen’s baby has been safely returned, Krag and an able bloodhound follow the petroleum trail, which leads them straight to Barra’s secret cavern. Krag arrives just in time to prevent Barra from handing the plans to Frank.