Stranger Things and Marvel Universe heavyweight David Harbour has ignited a firestorm of speculation by seemingly confirming a jaw-dropping plot twist in the upcoming Avengers: Doomsday extravaganza. In an interview with Variety, when asked to recite a random line from the movie out of context, Harbour dropped a potential game-changer - "You... are Tony Stark," he stated, adopting his Red Guardian character's Russian accent. Fans have seized upon this dialogue as a sign that the long-anticipated unveiling of Doctor Doom's MCU identity as the Iron Man alter ego will unfold within Avengers: Doomsday itself. Moreover, Harbour's line hints at a clever allusion to Downey Jr.'s iconic "I am Iron Man" declaration - his last words as Tony Stark in the Marvel Cinematic Universe so far. While it was widely expected that the MCU heroes would eventually uncover the truth about Doctor Doom's face, fans have been debating the timing of this revelation, with some suggesting it might be saved for Avengers: Secret Wars. The surprise here is Harbour revealing this dialogue now, as it will undoubtedly be a pivotal moment in the film. It's hard to imagine the shock that Tony Stark's former allies, such as Chris Evans' Captain America and Chris Hemsworth's Thor, will experience upon learning this bombshell. The MCU is set to explain Doctor Doom's similarity to Tony Stark by revealing that Doom hails from the Fantastic Four's reality - a different branch of the multiverse from the main 616 timeline. This suggests two versions of the same person could be possible. Doctor Doom first appeared at the end of The Fantastic Four: First Steps, but Avengers: Doomsday's latest trailer also hinted that Sue Storm had known Victor for some time. However, the exact origins of Doctor Doom's rise to power remain a mystery. In addition to Avengers: Doomsday, Harbour is also set to join the cast of the upcoming John Rambo reboot and will reunite with Stranger Things co-star Millie Bobby Brown in a new Netflix project.