After a research physician of brain abnormalities is murdered with a crossbow, Murdoch traces the hooded killer to the patients of his institute.

Detective Murdoch investigates the death of Sarah Bosen who apparently threw herself from her hospital room window and breaking her neck in the fall. Sarah was being treated by Julia Ogden for arachnophobia and had been making some progress in overcoming her fears. Murdoch however finds a dead spider in her room. The three remaining members of the treatment group begin to have their doubts about their therapy. Julia's colleague, Dr. Charles Linden, thinks her therapy is a waste of time and as far as he is concerned, Sarah's death proves him correct. He launches a formal complaint to have her research terminated. When her other patients are forced to face their phobias, Murdoch concludes that someone is trying to kill her patients. Meanwhile, Crabtree tries to save his pet spider from Dr. Grace's experiments and Dr. Ogden helps Murdoch overcome his own irrational fear.