Gordon Harker reprises his role of policeman Elk from 'The Frog' (1937) but here he has been promoted to Inspector. He leads the investigation against a criminal who radios in his instructions to his gang via a massive curio figure of a frog with beaming eyes. The gang are all known by numbers and they carry out raids and shootings and explosions in London. Chicago Dale claims to be an American policeman visiting Scotland Yard to learn about their detection methods. Elk tries to check with Chicago Police but their Chief has just been murdered so there is no corroboration of Dale's story.
Mum Oak's riverside bar is a great setting with it's stable door type entrance a few feet away from the lapping water of the River Thames. Inside is a cupboard safe and a back door that leads to a yard and a cellar convenient to carry out secret activities. Mum Oaks (Una O'Connor) is a fence who is well known to Inspector Elk and she is in with other dubious characters like Captain Dutchy and Dandy Lane. She treats her brother Golly in a harsh manner and all of the people at Mum Oaks' place come across as 1930s contemporary updates of Dickensian characters.
Inspector Elk is usually attired in a grubby raincoat and carries a brolly which he uses to rap on doors. He never knows whether he can trust Chicago Dale or not. He is under constant threat from such things as an exploding phone box and poison gas vapors disguised as London fog and a booby-trapped river police boat. Fortunately he carries a gas mask when he is trapped in Mum Oaks' cellar that is filling with poison gas. This is a good entertaining mystery filled with incidents.