The imminent release of Singer’s considerably grittier sequel, X2, continues the director’s fascination with the darker side of human nature. It is set in a paranoid world in which civil liberties have been eroded and Swat teams stalk children.

The director’s darker leanings have set him in good stead with X-Men fans who applauded the first adaptation. Singer avoided cartoonish camp, opting instead to focus on the characters’ predicament and the implications of their world. The film made $300 million worldwide and, for better or worse, triggered the current wave of comic-book movies. But while the earlier film spent a long time introducing the extended cast of mutants, X2 concentrates more on the story’s civil rights subtext.

Singer has used two writers, Dan Harris and Mike Dougherty, who are still in their early twenties and steeped in the comic’s labyrinthine lore. “They’ve completely comprehended and executed the history, depth and the tricky politics of these characters and this universe,” he says.

One example of those tricky politics is an unexpected alliance between the X-Men and their former enemy Magneto, played with relish by Ian McKellen. In his second time in the role, McKellen says: “I’m attracted to X-Men because of the moral basis of it, which I think, frankly, is a bit more interesting than Spider-Man. Our movie is about politics, about what it’s like to live in the real world, even though it’s called a fantasy. We have a story worth telling.”

Over 40 years, the X-Men saga has evolved into one of the most detailed story arcs in comic history, with over 100 recurring characters and fiendishly convoluted continuity. But the characters have been hugely successful with close to half a billion X-Men comics sold.

In the early Sixties, Stan Lee and Jack Kirby’s X-Men always stood slightly askew. The team’s ever-changing line-up has included a blue-furred intellectual named Beast, and Nightcrawler, a God-fearing soul cursed with the appearance of a demon.

But what gave X-Men its distinctive tone was the way it mirrored contemporary social unease in America. Despite their efforts against super-villains, the X-Men found themselves hounded as freaks and despised by the people they were trying to protect. That these heroes were viewed as subhuman rather than superhuman was, in 1963, an unprecedented twist. This outcast status rapidly became the comics’ central thread, and throughout the following decades writers developed the series’ racial and sexual metaphors. Also significant for the comics’ legion of adolescent readers, the stories described a generation gap of epic proportions.

At the saga’s heart lies the political clash between the X-Men’s mentor Xavier (played in Singer’s film by Patrick Stewart) and his rival, Magneto. Xavier is an optimist in adversity, with liberal aspirations and a belief in social progress. Magneto’s view of human nature is rather less charitable — though this misanthropy is not without foundation.

While comic-book bad guys are typically motivated by some inexplicable megalomania, Magneto’s militant philosophy is informed by a childhood spent in a concentration camp. Few villains have such an uncomfortably persuasive line of argument: “Doesn’t it ever wake you in the middle of the night, the thought that one day they will come for you . . . and your children . . . take you all away?” If Xavier is the mutants’ Martin Luther King, then Magneto is their Malcolm X.

During the Sixties, the comic-book industry watchdog proscribed any explicit discussion of race and sexuality. Yet the scenario of Xavier’s School for Gifted Youngsters introduced a generation of young readers to questions of prejudice and intolerance in a brave and adult way. If such claims sound grandiose, it is worth bearing in mind that the X-Men first appeared in September 1963 — a month after King’s march on Washington, and at a time when “Whites Only” signs were still in use.

With a $150 million budget (double that of the first film), Singer has noted the studio’s growing confidence in the X-Men franchise: “There was more time, more money and more freedom. And that allows me to make the type of film I enjoy making, a bit more my style. I think this is going to be a much bigger, better movie (than the first) and a little darker.”

• X2 is released in the UK on May 2

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