In the dead of night, a watchman is brutally attacked and with his dying breath cries out, "The Green Finger!" It is the latest in a series of robberies to take place that have left Scotland Yard mystified, and with no other choice but to call upon the expertise of Detective Paul Temple. Aided by the beautiful journalist Louise Harvey – affectionately known as Steve – the duo discover that this is not the first victim to warn of the dangerous and elusive 'Green Finger'... who or what is it? The pair must work together to solve the deepening mystery.