Until late last year, GameStop was a typical and not very successful corporation. The company sold videogames through a chain of retail outlets and lost money on every sale. But its stock caught the interest of small investors who traded on Robinhood, a mobile trading app, and the stock began to levitate.

From single digits in October 2020 the stock price doubled to 20 late last year. Then, over a few manic days in January, it vaulted “like a lid flying off a pot,” as Ben Mezrich puts it in “The Antisocial Network.” It went up to 77, then 148, then 348 and then an intraday high of 483—at which point GameStop was worth more than $30 billion. Briefly, it was the most heavily traded issue on the stock market.

The source of the mayhem was, to borrow from the book’s subtitle, “a ragtag group of amateur traders.” Few of the devotees who flocked to GameStop thought of themselves as even armchair security analysts. They were infected by crowd psychology and, in some cases, driven by the hope that the high price would punish well-to-do short sellers. The perils of short selling—betting on borrowed stock to decline—are nothing new. They were explained by Daniel Drew, the 19th-century robber baron: “He who sells what isn’t his’n / Must buy it back or go to pris’n.” But using social media to attack the shorts is assuredly new.

Mr. Mezrich, the author of bestsellers on topics ranging from the origins of Facebook to beating the odds at Las Vegas, tells the story of GameStop through the eyes of an array of characters, especially small investors who had little or no previous experience in the stock market.

The central figure is Keith Gill, a 30-something truck driver’s son and videogame aficionado who livestreams from his basement under the handle Roaring Kitty, explicating his view that GameStop was undervalued. His videos were accompanied by posts on the Reddit message board under the somewhat more assertive handle “DeepF—ingValue.”

Mr. Gill invested for not uncommon reasons: The stock was cheap, and he saw potential for the company to reinvent itself. Maybe he was right, maybe not. Collective madness often begins with a particle of sanity. Other traders read his posts, and copycat buying drove the stock higher. At some point, the GameStop investors focused on the fact that a large short position was outstanding and that, with every dollar the stock rose, the short sellers were losing money.

Valuation now no longer mattered. The aim of investors was to engineer a “squeeze”—that is, cause the shorts sufficient pain to compel them to repurchase, creating a vicious upward spiral. The small investors were egged on by celebrity touts such as Elon Musk and Jim Cramer.

Despite the depiction of the battle as David vs. Goliath (an idea swallowed whole by Alexandria Ocasio-Cortez), Wall Street pros also invested in GameStop. Even Robinhood’s everyman business model—commission-free trading, no minimum balances—depended on selling order flow to big market makers. Its mission to democratize Wall Street was really a rebranding of the strategy pursued by Charles Merrill, who after the debacle of the Depression sought to extend Wall Street beyond the rich to the ordinary American home.

Mr. Mezrich is impressed by the seeming ideological commitment of the investors on Robinhood. Many participated in a coordinated attack on the shorts, even when such a move meant buying at seemingly unrealistic prices. And many were animated, to judge from their social-media posts, by resentment of Wall Street elites. The war got nasty even by the standards of online trash talk. One of the biggest shorts was a fund named Melvin Capital, managed by Gabe Plotkin. Seizing on the Jewish name, a GameStop investor posted: “It’s very clear we need a second holocaust, the Jews can’t keep getting away with this.”

Even when the price hit the stratosphere, retail buyers professed not to be worried. They would “never” sell; they weren’t concerned with the possibility of losing money. “Oh im [sic] fully aware that I may end up a bagholder,” went one post. “But it’s worth being a bagholder to stick it to those Wall Street f—s who’ve gamed the system for so long at our expense.”

To Mr. Mezrich, such fulminations suggest that a revolution is a-coming. His thesis is vented in excited metaphors. The “pillars” of Wall Street are shaking; Melvin Capital faces an “existential moment” (which, actually, it survived); angry traders constitute a “millennial version of the French Revolution.”

A little of this gas comes from investors; most of it is supplied by Mr. Mezrich. “The Antisocial Network” is built on scenes that the author has re-created; quotation marks, in the main, are conveniently absent. He writes of one novice but gung-ho investor, who worked in a hair salon: “She believed something deeper was happening.” Did she say that? Is it a paraphrase? Is it what Mr. Mezrich thinks she believed?

Even for investors who felt part of a crusade—the author calls them a “tribe”—sentiments expressed in the heat of a buying mania are not necessarily predictive of how people will behave later, when their capital is at risk.

Mr. Mezrich opines that GameStop represents the culmination of a populist movement and that, in the future, the stock market might lose all connection to the fundamentals, like “an untethered balloon.” But the thing with a short squeeze is that, once the shorts capitulate (as they did in GameStop), there are few folks left to buy. GameStop’s stock remains many multiples above its pre-mania level, but it has fallen from its peak by more than half. The fundamentals are not dead yet.

As for Mr. Mezrich’s revolution, it may have already occurred. Over the past two decades, the small investor in index funds has consistently outperformed the pros, including hedge funds and mutual funds. Compared with the lure of long-term appreciation, speculative crusades may have diminishing appeal.

Mr. Lowenstein writes at Rogerlowenstein.substack.com.