‘A case of obsessional neurosis’ discusses the case of a patient that Sigmund Freud first met in 1907. This particular and complicated patient was known as ‘The Rat Man’. The patient faced a desperate internal situation and was also incredulous, as his analysis unfolded, that he could be so encumbered by thoughts and driven to actions that defied rational sense. Freud went to extraordinary lengths to grasp the ideas behind his patient’s apparently nonsensical activities, to trace his psychic history, and to understand how obsessional doubts governed his life. Freud’s 1923 model of the mind and its three agencies—the superego, ego, and id—are also considered.