Writer-director Joss Whedon said he wanted to "perform a little open heart surgery" on the Avengers, and "let the audience experience them on a more personal level than they ever have before."
In "Age of Ultron," Marvel's super hero worlds come crashing together as the super heroes tackle a complex villain in Ultron (James Spader), spawned out of artificial intelligence and evolving into a megalomaniac.
With help from twins Pietro (Aaron Taylor-Johnson), known as Quicksilver for his super speed, and Wanda (Elizabeth Olsen), the Scarlet Witch with thought-altering magic powers, Ultron makes the Avengers at times turn on themselves.
Robert Downey Jr., Marvel's Iron Man and patriarch of the franchise, said given the present day "world stage," now is the time for the Avengers to get real.
"This is where you have to actually start dealing with the ramifications of saving the world," he said.
BUSINESS OF GOING DARK
The trend toward sombre super heroes has already proved to be a winner at the box office for other studios. The 2013 Superman film "Man of Steel," from Time Warner Inc's Warner Bros, made more than $660 million worldwide. The studio's "Dark Knight" Batman trilogy grossed more than $2 billion globally between 2005 and 2012.
Olsen said "Age of Ultron" allows audiences to see the "most human versions" of the seemingly invincible Avengers.