A man is found dead in the forest. He has needle marks on his arm and marks around his wrist. He was writing an expose on the "Illuminati" a secret society that has members in high positions.
Inspector Sullivan investigates the case with a young new policeman who is French and very smart. The policeman finds a lot of clues Sullivan does not see.
The Illuminati members hide crimes and protect each other. Inspector Sullivan is relieved from his duty because a top Illuminati member Mr Greensleeves is friendly with police Commander Busby.
The young police man that helped Sullivan is stabbed in the evidence room and Inspector Sullivan is arrested.
The story has lots of deceptive things going on. The autopsy on the writer was not true the man was murdered and did not commit suicide like documented. The autopsy showed signs of a truth serum drug used on him.
Father Brown is funny especially his facial expressions. Him and his friends, Mrs McCarthy, Sid and Lady Felicia help Inspector Sullivan but he is still does not want their help or is thankful when they help him.
In the end a expose that the writer wrote is what these Illuminati members are seeking to keep out of the eyes of the public.