When Mr. Lomas visits the Chief Constable of Burnham and describes his symptoms, Sir Wilfred Burrows believes that his visitor suffers from nothing more serious than nerves. Later that day Mr. Lomas's body is recovered from the water at Willow Lock; yet death is not by drowning.

Sir Wilfred recounts the interview to Inspector Knollis, who, realizing the significance of the symptoms, is satisfied that Mr. Lomas is a victim of cocaine poisoning. With characteristic energy he sets about the task of unmasking the murderer.

In this gripping story of a cunning murderer brought to justice by brilliant, logical reasoning, the solution is skilfully yet legitimately concealed to the last.