On screen, they’re Earth’s mightiest heroes. But in Hollywood, they were has-beens and never-weres. That is, until Marvel got a hold of them.

“Avengers: Age of Ultron” kicks off the summer movie season May 1 with a whirlwind of superheroes joining forces to once again save the world and shatter box-office records. The director and several key stars in the ensemble cast either kick-started their film careers with Marvel Studios (Chris Hemsworth as Thor, Chris Evans as Captain America, director Joss Whedon) or staged a dramatic comeback (Robert Downey Jr. as Iron Man). James Spader, best known as a quirky TV actor, is Marvel’s latest addition, voicing the villainous Ultron Prime.

The next four months could mark the most lucrative summer in Hollywood history, thanks to a wave of follow-ups to major franchises. After “Avengers,” some of the most anticipated entries include “Mad Max: Fury Road,” “Jurassic World,” “Terminator: Genisys,” “Mission Impossible: Rogue Nation” and the “Despicable Me” spinoff “Minions.”

Those moviegoers who prefer original stories over digitally created dinosaurs will also have a surprising surfeit of choices, as Hollywood has learned that summer can be a smart time for sophisticated counterprogramming. Two of the most celebrated movies from January’s Sundance Film Festival, the dark comedy “Me, Earl and the Dying Girl” and hip-hop comedy “Dope,” will both be released in June. Later in the summer there’s the boxing drama “Southpaw,” starring Jake Gyllenhaal, and Meryl Streep as an aging rock star in “Ricki and the Flash.”

But in a summer with 18 sequels, reboots, spinoffs and comic-book adaptations, it’s the second “Avengers” that will set the tone. The film also demonstrates a new casting calculus in Hollywood, in which it’s not the actors who make the character, but the character who makes the actor.