The first sign of trouble for hedge fund wunderkind Gabe Plotkin came in late October: A poster on Reddit's popular "wallstreetbets," or "WSB," forum was taking aim at his wildly successful investment firm.

"GME Squeeze and the demise of Melvin Capital," wrote the user, Stonksflyingup, referring to GameStop Corp. (stock ticker: GME) and Plotkin's $12.5 billion firm. Before long, veryforestgreen weighed in: "Melvin Capital New Short Attack." Then, greekgod1990: "Melvin vs WSB! And GME to the moon."

So it was that the tables turned on Wall Street—and a hedge fund star suddenly found himself at the mercy of the day-trading Reddit bros who have become one of the most powerful, if improbable, forces in the stock market today. The attack on Plotkin's six-year-old Melvin Capital shifted the balance of power in ways that would have seemed unimaginable only months ago. By Wednesday, the firm had capitulated to the amateurs and covered its GameStop short.