The story opens with a meeting at Scotland between the Commissioner of the Metropolitan Police; Sir Graham Forbes, Chief Inspector Dale and Superintendent Harvey. The topic of conversation is a mysterious series of jewel robberies. The latest in the series has just occurred in Birmingham; Dale believes they are all unrelated, but Harvey is inclined to agree with Inspector Merritt (a local officer - first on the scene in this case) who is of the opinion that there is a criminal organisation at work, and that this gang committed the earlier robberies. A nightwatchman has died after being chloroformed. He was semi conscious when Harvey arrived, and all he said was "the Green Finger..."
It transpires that a suspect in the earlier robberies dying words were also "the Green Finger...". What is it? Sir Graham tells Dale and Harvey that this is something they must find out.