Returning director Joss Whedon pits the superhero gang against their own shortcomings as they take on their ultimate enemy - but also plays with the chemistry that developed between them in their first run-out.
The original 2012 The Avengers became the third-highest grossing movie in cinema history, minting over $1.5 billion at the box office, only beaten by Avatar (2009) and Titanic (1997).
The pressure on Mr Whedon was therefore enormous. But the 50-year-old director said he drew the most inspiration from amidst the first film's success, and from the interplay between the main characters.
"I looked for what little moments are there between these characters that I haven't gotten to do yet, what conversations they haven't had yet, what haven't I shown?" Mr Whedon told reporters in Los Angeles.
Relationships are indeed richer between Iron Man/Tony Stark (Robert Downey Jr), Captain America (Chris Evans), Thor (Chris Hemsworth), Black Widow (Scarlett Johansson), Hulk (Mark Ruffalo) and Hawkeye (Jeremy Renner).
There is also more humour to temper filmgoers' pure adrenalin rush.
"The main thing was to make sure that everybody had their moment, that it's all connected to the movie, to the main thing," said the filmmaker, who spent hundreds of hours editing the movie.
The result is two hours of turbo-driven action, superhero fist-fights and technology at the service of a story full of surprises, which USA critics have already hailed as a huge hit.
Industry journal Variety called it a "super-sized spandex soap opera that's heavy on catastrophic action but surprisingly light on its feet, and rich in the human-scale emotion that can cut even a raging Hulk down to size
James Spader, known for his roles in TV shows Boston Legal and The Blacklist, plays ultimate baddie Ultron, a robotic assassin able to reproduce himself and who is determined to annihilate The Avengers.
He wins the unexpected backing of twins Wanda and Pietro Maximoff, also known as Scarlett Witch and Quicksilver and played by Elizabeth Olsen and Aaron Taylor-Johnson).
Jarvis/The Vision (played by Paul Bettany) makes his entry as a counterweight to the power of Ultron, who emerges as a kind of ghost in the Stark Industries machine.
"They embody a little bit of him (Tony Stark)," said Mr Whedon.
"But I do see them as both sides of the same coin, sort of a core between the two of them. There's something beautiful about the fact that they see the same thing and react differently."
The climax of the film closes certain storylines from the first film, and opens new horizons for those to come, The Avengers: Infinity War, the third part of the saga that Marvel is dividing into two movies.
The first part will release on May 4, 2018, followed by the second on May 3, 2019. Both will be directed by filmmaking brothers Anthony and Joe Russo.
But it is unknown what role new Marvel characters could play. "There are so many different directions to go," said Mr Johansson. "We'll see," he added.
Avengers: Age of Ultron will be released in the United States on May 1, a week or two after it is out around much of the rest of the planet.