One Avengers: Endgame Line Is The Key To Doomsday According To MCU Theory

Published Aug 12, 2026, 10:00 PM EDT Liz Declan is a Lead Writer for ScreenRant, primarily covering movies. She is a massive fan of Star Wars, the MCU, and Supernatural. In addition to writing, Liz loves attending and covering conventions and interviewing actors. Find her on TikTok and Instagram @va.va.vera With Spider-Man: Brand New Day having been in theaters for nearly two weeks now, MCU fans are already beginning to shift their focus to what is sure to be one of the biggest movie events in the entire franchise thus far, Avengers: Doomsday. Doomsday will be monumental for myriad reasons, not only because it will be the first Avengers movie since Avengers: Endgame, which marked a true end of an era for the MCU, but also because the Doomsday trailer has already revealed that plenty of twists and shocking reveals are in store for audiences in this upcoming movie. Among the biggest questions that Doomsday has to answer are how and why both Chris Evans as Steve Rogers is back and Robert Downey Jr. is back not as Tony Stark/Iron Man but as the formidable Doctor Doom. These questions actually predate even the most recently released trailer, as it has been known for some time now that both will be back. There are a range of theories regarding how these returns are happening, from RDJ playing a Tony variant as Doctor Doom to Doomsday's Steve Rogers secretly being Loki. The actual explanations for these respective returns will no doubt remain tightly under wraps until the movie hits theaters, but given that Doomsday's release date isn't until December 18, that is a rather long time to wait. Thankfully, it's possible that the MCU has left hints for fans as to where this story could be going, and one blink-and-you-miss-it moment from Endgame may actually have contained the answer all along. Tony Stark Tells Steve Rogers He'll "See" That Time Messes Back The primary focus of the first half of Endgame is to bring back everyone who Thanos snapped away in Avengers: Infinity War, but in order to do so, the Avengers had to mess with time itself. Luckily, Tony Stark really is a genius, true to his word, so he was able to solve this massive puzzle and give the Avengers the chance to go back in time, collect the Infinity Stones, and bring their lost loved ones back. However, as can be expected in an MCU movie, things don't exactly go according to plan. Tony's newfound knowledge of how to time travel worked, and Hulk was even able to use the Infinity Stones to snap everyone back. During their travels in the past, though, the Avengers inadvertently alerted Thanos to what they were doing, and he therefore arrived just after they had brought everyone back and began smashing the Avengers compound into rubble, nearly killing everyone inside just after this triumph. As the heroes are attempting to recover and find one another, Tony goes to Steve and tries to give him his shield. When Steve, understandably, asks what happened, Tony replies, "If you mess with time, it tends to mess back. You'll see." On the one hand, this warning makes sense contextually. Tony had warned the Avengers that attempting time travel was incredibly dangerous and could have implications they didn't intend. At the time, this seemed like all that Tony's comments really meant. However, in the context of Doomsday, wherein Robert Downey Jr. is back as Doctor Doom, that "You'll see" in particular has led to brand-new theories. Could The Time Heist Have Caused Doomsday? With so little context as to how Steve Rogers is back in Doomsday and why Doctor Doom is played by the actor who played Tony Stark, it makes sense that audiences have begun suspecting that Endgame's time heist may have something to do with these returns. After all, even before Tony's comments to Steve about time messing back seemed as ominous as they do now, the risk of messing with time was already clear in Endgame. Yet, this scene now adds another layer, suggesting that this may very well have been a very early hint of what was to come. As of right now, we have next to no idea what the connection between Tony Stark and Doctor Doom will be, if anything—although, surely, it will be something. Doom being a Tony variant certainly seems high on that list of possibilities, but Doomsday could go in any number of shocking directions. Perhaps this line will retroactively be revealed as Tony Stark knowing what was coming in Endgame or will somehow be linked to Doom himself. Given that changes are being made to Endgame to bridge the gap to Doomsday, it really feels like nothing is off the table. Doomsday Has Some Serious Explaining To Do It's very easy to imagine that it was indeed the time heist that has led to these confusing returns, and there are frankly many ways Doomsday could use the Avengers' decision to mess with time as a way to explain how these actors are back. Whether it ends up being linked to that now rather unsettling Endgame line or not, though, there is clearly so much that Doomsday has to answer for. There is currently plenty of very reasonable concern within the fanbase that these returns are simply low-hanging fruit. That is, many are concerned that Robert Downey Jr. and Chris Evans are being brought back because Disney knows these two actors will guarantee viewers flocking to theaters, which, some worry, could mean this is a way to cover up other issues with the plot. Of course, that is all speculative. For now, details of the plot are very few and far between, and it seems just as likely that Doomsday is going to pull off some incredible reveal that brilliantly explains why RDJ is now Doctor Doom (and, hopefully, why Steve was fine with abandoning Bucky). It's an understandable fear regardless, particularly in light of the MCU's releases following Endgame. It's not a secret that the franchise has struggled in the time since. Even with some massive wins like Brand New Day, the MCU has arguably had as many low points as it has high points since the Infinity Saga concluded. It does, admittedly, seem like Evans and Downey Jr. returning could just be a way to get audiences excited again. However, if Avengers: Doomsday does build on this Endgame line and explain these returns well, it could be exactly the twist the movie needs. Avengers: Endgame 9/10 Release Date April 26, 2019 Runtime 181 Minutes Avengers: Endgame, directed by Anthony and Joe Russo, concludes the Marvel Cinematic Universe's Infinity Saga. It follows the remaining Avengers as they attempt to reverse the destruction caused by Thanos. Avengers: Doomsday Release Date December 18, 2026 Runtime 165 Minutes Cast Vanessa Kirby Sue Storm / Invisible Woman Johnny Storm / Human Torch Ebon Moss-Bachrach Ben Grimm / The Thing Avengers: Doomsday is the fifth Avengers film in the Marvel Cinematic Universe and will bring together heroes, new and old, to face off against Victor von Doom - played by the returning Robert Downey Jr. The upcoming film notably marks the beginning of Phase 6 of the MCU.