NEW YORK — (CNN) Fans of “To Kill a Mockingbird” will have read about three-quarters of the first chapter of Harper Lee’s hotly anticipated second novel before their stomachs drop.

We’re not going to reveal the stunning plot development right here, because everybody hates spoilers, right? But there will be some pretty big hints about what it involves, so read on at your own risk.

The first chapter of the novel, “Go Set a Watchman,” was published online early Friday ET by The Guardian in the U.K. and The Wall Street Journal in the U.S. The book comes out Tuesday.

Reactions swiftly followed on social media, with many readers flabbergasted by the big reveal concerning Jem Finch, a central character in “To Kill a Mockingbird.”

“Wow, Harper Lee just broke my heart this morning with one sentence,” tweeted book seller Joe Whittington.

The start of “Go Set a Watchman” picks up the story of Jean Louise “Scout” Finch, the young girl who narrates her adventures in the rural South in “To Kill a Mockingbird.”

But now she’s a grown woman in the 1950s, riding a train from New York back to her childhood home of Maycomb, Alabama, the setting for “To Kill a Mockingbird.”

This time, Lee tells the story in the third person, not in Scout’s voice.

The first chapter is still rich with the author’s wry observations about human nature, with turns of phrase like, “If you did not want much, there was plenty.”

Scout has become Jean Louise, transforming from “an overalled, fractious, gun-slinging creature into a reasonable facsimile of a human being.”

Besides the gut-punch revelation about her older brother, Jem, we also learn about a romantic interest for Jean Louise and health problems faced by their aging father, Atticus.

There’s even an eyebrow-raising detail about what Jean Louise likes to wear in bed.

But the first chapter didn’t persuade everyone that the novel, which is reportedly the most pre-ordered book since the Harry Potter series, will live up to the hype.

One Twitter user said she found the excerpt “problematic,” another was unimpressed with its description of Jean Louise, a former tomboy, as “easy to look at.”

Lee’s publisher announced in February that the 89-year-old author would be publishing the second book, which she actually wrote before “To Kill a Mockingbird.”

“In the mid-1950s, I completed a novel called ‘Go Set a Watchman,’ ” Lee said in a statement released by her publisher. “It features the character known as Scout as an adult woman, and I thought it a pretty decent effort. My editor, who was taken by the flashbacks to Scout’s childhood, persuaded me to write a novel (what became ‘To Kill a Mockingbird’) from the point of view of the young Scout.”

The manuscript of “Go Set a Watchman” was rediscovered last year, the statement said, although The New York Times reported last week that it may have been found earlier, in 2011.

Questions were also raised about whether Lee, who lives in an assisted living facility in Monroeville, Alabama, was in a fit state to knowingly consent to the publication of the novel.

For decades after “To Kill a Mockingbird” came out, Lee had steadfastly refused to publish another book.

State officials in Alabama who investigated the situation said in March that they had found that the author did wish to publish “Go Set a Watchman.”

In the book, Jean Louise is “forced to grapple with issues both personal and political as she tries to understand her father’s attitude toward society, and her own feelings about the place where she was born and spent her childhood,” according to the statement.

“To Kill a Mockingbird,” which Lee wrote after she moved to New York, made her name after its publication in 1960.

The book, which addresses the problem of racism in the South, won the Pulitzer Prize and was made into a beloved 1962 film.

Gregory Peck won the Oscar for best actor for his portrayal of Atticus Finch.

It’s a mainstay of high school reading lists and, as of 2006, had sold more than 30 million copies. Until now, it had been her only published novel.

The-CNN-Wire ™ & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved. (Photo: Wikimedia Commons, The White House)