Using The JohnLock Conspiracy (TJLC), developed by the fandom of the BBC television series 'Sherlock' (2010), as an exemplar, we analyze how the functionality of Tumblr supported the development of a fandom eschatology. In this instance, eschatology is not religious but secular: fans claimed to know what the final end of the Sherlock series was to be, and they interpreted various signs as indicating that this would happen. The infrastructure and interactive design of Tumblr as a platform creates a foundation permitting fan group radicalization. Because of Tumblr's infrastructure and gratification system, forming a tight-knit group is difficult. By developing and using eschatology as a belief system, fans create boundaries, decide membership and proper behavior, and enable the policing of other fans.