The "Frog with the Mask" spreads fear and anxiety within the streets of London. The creature is hidden under a wide clergyman's robe, its face covered up and only two enormously frightening, gleaming eyes can be seen. Together with members of his gang the "Frog with the Mask" hunts down innocent victims. Scotland Yard's Superintendent Elk is full of activity, as there are absolutely no traces on the crime scenes - only the wrists of the murdered bear a visible trace: the black "Frog Gang" mark. But - Then there is also Private Detective Richard Gordon together with his butler who are trailing down the "Frog with the Mask" as well...