There are theoretical challenges in distinguishing obsessions from overvalued ideas (OVIs) and delusions. The latter can be defined by its rigid nature and unusual content, but obsessions and OVIs are also reported with such characteristics. Clinically, what are the implications when this distinction is not entirely clear? In this case study, these issues are explored with a man who presented with commonly reported symptoms of obsessive compulsive disorder (OCD) and a belief about transforming into a giant fly. Clinically meaningful gains were made following a course of 17 sessions of CBT, providing some additional support for the effectiveness of CBT for abnormal beliefs within OCD.