Fantastic Four #1
Four friends — scientist Reed Richards, his fiancée Sue Storm, her brother Johnny, and pilot Ben Grimm — gain superpowers after exposure to cosmic rays during an unauthorized rocket flight. Rather than forming a classic crime-fighting team, they bicker, argue about money, and struggle with the psychological impact of their transformations, particularly Ben Grimm's monstrous rock body. Their dynamic is explicitly that of a dysfunctional family rather than a heroic team.
Fantastic Four #1 launched the "Marvel Age of Comics" by bringing psychological complexity and interpersonal conflict to superheroes, fundamentally changing what the genre was allowed to be. The "superheroes with real problems" concept that Lee and Kirby established in this issue became the DNA of all subsequent Marvel characters — Iron Man's alcoholism, Spider-Man's guilt, the X-Men's persecution — and eventually the entire MCU franchise.
Stan Lee claims he was about to quit comics when publisher Martin Goodman told him that DC's Justice League of America was selling well and demanded a team book in response. Lee decided to create the book he actually wanted to make, producing characters defined by scientific curiosity and emotional reality rather than aspirational perfection, launching Marvel's dominance of the superhero genre.
Dr. Fredric Wertham's book "Seduction of the Innocent" triggered a Senate investigation that led to the self-regulatory Comics Code Authority, effectively ending horror, crime, and romance comics and consolidating the market around Code-approved superhero and Western stories. The vacuum left by these dead genres was eventually filled by the underground comix movement of the late 1960s.