Harry Potter and the Goblet of Fire by J.K. Rowling serves as the definitive "loss of innocence" moment in the series, shifting from whimsical mysteries to a dark, wartime narrative. The plot centers on the dangerous Triwizard Tournament, where Harry is mysteriously selected to face deadly tasks, ultimately culminating in the tragic return of Lord Voldemort in a suburban graveyard. This pivotal installment marks the end of childhood, introduces a broader global wizarding community, and explores themes of prejudice, setting the stage for the high-stakes rebellion in the later books.