A gang of London criminals think that anything they do is justified, however criminal it may be. They plot to steal diamonds from a jewellery business, having been invited to do so by an unnamed member of the office staff. However, one member of the gang has had enough of crime, and has contacted the police, acting as an informer. The police Superintendent who takes over the case has doubts as to whether he should stop the gang before the offence, or should let them go ahead with it, and then arrest them. The robbery goes wrong in many ways, but so does the police ambush when the Superintendent¿s wife intervenes.