With all the debate brewing over the origins of Harper Lee’s novel “Go Set a Watchman,” the biggest bombshell turned out to be an explosive plot twist that no one saw coming.

Atticus Finch — the crusading lawyer of “To Kill a Mockingbird,” whose principled fight against racism and inequality inspired generations of readers — is depicted in “Watchman” as an aging racist who has attended a Ku Klux Klan meeting, holds negative views about African-Americans and denounces desegregation efforts. “Do you want Negroes by the carload in our schools and churches and theaters? Do you want them in our world?” Atticus asks his daughter, Jean Louise (the adult Scout), in “Watchman.”

The revelation will probably alter readers’ views of “Mockingbird,” a beloved book that has sold more than 40 million copies globally and has become a staple of high school curriculums. It could also reshape Ms. Lee’s legacy, which until now has hinged entirely on the outsize success of her only novel, published 55 years ago.

It is also certain to spur debate about the character of Atticus and his moral integrity in “Mockingbird,” which made him a cultural icon whose influence transcended literature, inspiring generations of lawyers, teachers and social workers.