On 10 October 2004, an improvised explosive device was detonated in bush land in the vicinity of Mount Disappointment on the outskirts of Melbourne, Australia. A relatively small device, it was assembled and detonated by one of the two men present, a Victoria Police officer and undercover operative known as Security Intelligence Officer 39, or SIO39. The other person was the leader of a homegrown terrorist cell, who in the months preceding had assembled a group of a dozen individuals who became the subject of Australia’s largest counterterrorism investigation known as Operation Pendennis. This article, which is based on in-depth interviews with SIO39, provides unique insights into a range of activities and behaviors peculiar to this cohort. Commencing his association with the group early in its development, SIO39 was privy to some of its key evolutionary stages, from a collection of individuals meeting more or less in the open, to a clandestine body that clearly harbored terrorist intent and undertook a number of overt acts to advance its violent objectives.