Autocrats often rely on their secret police to stay in power. How can an autocrat ensure that this organization does in fact repress his opponents? And in what circumstances does the secret police defect and refuse to repress the opposition? I present a dynamic model where an autocrat is threatened by a mass opposition movement and uses the secret police to repress it. I examine the conditions under which secret police agents will repress even when they have no interest in the survival of the autocratic regime. I then examine how the interaction with opposition activists may affect the agents’ decision to repress, and how the opposition may choose tactics to make the secret police’s defection more likely.