In the wake of his Avengers: Endgame farewell in 2019, the original MCU hero has largely remained behind the camera and off the silver screen.
That is, until Christopher Nolan cast the Iron Man actor in his upcoming, star-studded biopic about the father of the atomic bomb, J. Robert Oppenheimer.
Set to release June 21, this World War II & Cold War-era movie stars Cillian Murphy as the film's title star alongside Emily Blunt, Matt Damon, Florence Pugh, and Downey Jr. as the film's potential antagonist, Lewis Strauss.
Robert Downey Jr.'s Oppenheimer Character: Hero or Villain?
History knows J. Robert Oppenheimer as the theoretical physicist who helmed the World War II development of the atomic bomb, leading to the United States dropping two atomic bombs in Japan in 1945.
Much of this is what Christopher Nolan's Oppenheimer looks to explore.
However, Oppenheimer's story, along with his own professional explosion, isn't complete without Lewis Strauss.
After the Soviet Union tested its own atomic bomb in 1949, Lewis Strauss championed the further development of thermonuclear weapons, something Oppenheimer personally opposed.
Marketing for Oppenheimer has already shown Robert Downey Jr. as Strauss saying, "Truman needs to know what's next," possibly in reference to the Hydrogen bomb.
Apart from professional disagreements, Strauss came to personally dislike and even distrust Oppenheimer for various reasons and even believed him to be a Communist spy.
This was made all the worse by knowledge of Oppenheimer's relationship with the Communist and American psychiatrist, Jean Tatlock, played in Nolan's film by Black Widow's Florence Pugh.
While playing an obstacle for a film's protagonist is somewhat new ground for the Marvel star, winning a fight is not. And, that's exactly what Robert Downey Jr.'s character is expected to do by the film's end.
How Will Robert Downey Jr. Play His Bombshell Role?
Christopher Nolan's Oppenheimer is based on the 2005 book about the real-life scientist titled American Prometheus, a play on the guilt-ridden Titan god who stole fire from the gods and gave it to humanity through technology.
While trailers for the June film show a driven, if not reckless, J. Robert Oppenheimer, it's important to remember that time is always an element of Nolan's films.
Cillian Murphy's scientist may be driven to detonate an explosion at the start of the film, but Robert Downey Jr.'s Strauss may be driven to detonate the burdened Oppenheimer's career by the film's end.
Also, given the presence of Florence Pugh's Tatlock, and the rising fear of the Soviet Union, Cold War paranoia may be the very button Strauss strikes in removing Oppenheimer from the world's stage.
While Strauss serving as Oppenheimer's antagonist appears to be a given, it will be interesting to see whether Downey plays him as a villain or, like another Oppenheimer, someone who believes he's saving the world.
Oppenheimer arrives in theaters on June 21.