Last October, an anonymous user, known simply as Q, started posting cryptic messages on the controversial message board 4chan—the common theme being that President Trump is a secret genius and his opponents, namely Hillary Clinton and Barack Obama, are evil. Q reportedly claimed to be getting this information directly from the government, thanks to tops-secret, “Q-type” security clearance. There has been little—if any—hard evidence to support Q’s musings. But over time, thousands of people started to believe them—or at least, to acknowledge they might be real. And they became the foundation of a wide-ranging conspiracy theory, known as QAnon, that...