Wild conspiracy theories have a way of seeping into public discourse these days, thanks in part to the divided nature of U.S. politics, the growth of websites that actively promote them, such as InfoWars, and the capacity for fake news to spread virally on social media without any fact-checking or oversight. Enter “Q.” In late October, just days before a different InfoWars-inflated conspiracy—about anti-fascist protesters plotting a civil war—was about to fizzle, a user identified as Q on the imageboard website 4chan started posting vague, portentous messages related to an approaching “storm.” The user claimed to be a high-level government...