Dr. Cameron is called out urgently to find his patient, Roderick McDougall, unconscious on the floor. He is baffled as to the cause. One curious aspect of the case is that McDougall seems to fall ill only on Tuesdays. Eventually Cameron thinks he has solved the puzzle, but as he proceeds with the case he finds himself in conflict not only with Dr. Snoddie, but with his partner Dr. Finlay as well.