David Harbour appears to have incidentally spilled a major spoiler from the much-anticipated Avengers: Doomsday.

Out in theaters December 18, the new Marvel crossover will welcome back Iron Man’s Robert Downey Jr. to the universe in a new role as villain Victor von Doom/Doctor Doom.

Downey Jr. is best known to Marvel fans as Tony Stark/Iron Man, a character he played in 10 different movies, including Iron Man (2008), Captain America: Civil War (2016), Spider-Man: Homecoming (2017) and four different Avengers films. His superhero ultimately died in 2019’s Avengers: Endgame.

When his casting as Doctor Doom was announced in December 2024, the news left fans baffled, as he had spent years defining himself as heroic Tony Stark/Iron Man, only to return as the franchise’s villain.

Trailers and announcements had also offered little insight into how the face of Iron Man could now be Victor von Doom, sparking speculation that Doctor Doom might be a multiversal Tony Stark variant.

And it seems that Harbour may have just endorsed those theories. In a recent interview with Variety, the Stranger Things star was asked to deliver a line from the movie out of context.

“You… are Tony Stark,” he obliged, using his character’s signature Russian accent while pointing.

The moment may have been Harbour’s attempt to throw fans off with an entirely fictional quote. But, if it is indeed a line said by his super-soldier Red Guardian/Alexei Shostakov in the film, likely in reaction to Doctor Doom’s identity after he removes his mask, it could mean that Tony Stark exists as Doctor Doom in a parallel universe.

Or, in an even bigger — though less likely — twist, it could tease Tony Stark’s return.

The former theory wouldn’t be completely out of the question, as Marvel has previously introduced multiversal variants of other characters in projects such as Loki and Doctor Strange in the Multiverse of Madness.

It was also revealed in the recent Fantastic Four: First Steps (2025) that Doctor Doom is from Earth-828, a futuristic, 1960s-inspired alternate universe and original home to the Fantastic Four.

Directed by brothers Joe and Anthony Russo, Avengers: Doomsday will bring together a host of Marvel legends: Chris Evans, James Marsden, Rebecca Romijn, Ian McKellen, Chris Hemsworth and Patrick Stewart. They will be joined by newer Marvel actors including The Fantastic Four: First Step’s Vanessa Kirby and Pedro Pascal, Thunderbolts*’ Lewis Pullman and Florence Pugh, among others.