Warning: Possible SPOILERS for Avengers 4 ahead

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Avengers 4 may see Tony Stark travel back in time - and change Loki's personal history. It's generally believed that next year's Avengers sequel will be a time-travel film of some kind, with the Avengers attempting to rewrite history in order to prevent Thanos being able to erase half the life in the universe.

One of the strongest pieces of evidence for this is a range of set photos showing the film will revisit the Battle of New York. Set photos showed Tony Stark wearing the same Black Sabbath shirt he wore for most of The Avengers, and his face was bruised, suggesting this was shortly after the Chitauri invasion. More set photos showing Thor and Loki in their classic Avengers-era costumes, with Loki taken away in chains as in the first movie. A final batch of set photos revealed Captain America in his "old-fashionedAvengers uniform - standing next to an older Tony Stark and, of all people, Ant-Man.

Related: When Will The Avengers 4 Trailer Release?

But we may have missed a major clue in the set photos. Back in November 2017, images surfaced of Robert Downey Jr. on the set of Avengers 4 wearing a S.H.I.E.L.D. tactical vest. These generated a lot of speculation among fans, with some suggesting this was a glimpse of a timeline where Tony Stark had become director of S.H.I.E.L.D.. That really doesn't seem to be the case; rather, it looks as though Stark has traveled back in time and infiltrated S.H.I.E.L.D. in the aftermath of The Avengers.

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Loki captured by the Avenger with gag over his mouth 2012

There are two pieces of evidence for this. The first is that the set photos showing an older Stark in a S.H.I.E.L.D. uniform are in the same building we've seen Marvel shooting a long-haired Thor, with Loki in chains as at the end of The Avengers. The second, more subtle, detail is that Stark is wearing a S.H.I.E.L.D. uniform we've seen before. It actually appears in The Avengers, worn by the S.H.I.E.L.D. unit Nick Fury assigns to guard Loki on the Helicarrier. Presumably a similar team would be assigned to guard him after the Battle of New York as well. These uniforms include a face-mask, meaning that Stark would be able to conceal his identity with ease.

The fact that Tony Stark goes undercover like this is interesting by itself. It seems to imply that he's attempting to minimize his "temporal footprint," to carefully control the changes he makes to the timeline. This may be our most important clue as to what laws govern the time-travel in Avengers 4; the Avengers believe that time can indeed by changed, but they're trying to make only the more subtle adjustments.

This may imply that one of the most common Avengers 4 theories, that the Avengers are attempting to take the Infinity Stones out of circulation, may not be correct. After all, if Stark retrieves the Space Stone from this point in the timeline, he prevents Thor taking Loki back to Asgard, since the God of Thunder used the Tesseract to go home. Indeed, Odin was only able to repair the Rainbow Bridge when he was given the Tesseract. Meanwhile, taking the Mind Stone would mean neither Ultron nor the Vision were ever created. Removing either Infinity Stone at this point would be to make a dramatic change to the timeline, rewriting the histories of entire worlds. That doesn't fit with the apparent subtlety of Stark's strategy, with his quiet infiltration of S.H.I.E.L.D..

So what is Tony Stark's goal? It may be that, rather than retrieve an Infinity Stone, he's here with another agenda. Perhaps his true aim is to get to Loki, to either learn crucial information about Thanos from him or to actually impart knowledge the God of Mischief can use in the future. Right now, it's difficult to say; but just the fact Stark has gone undercover in Avengers 4 means we may not have worked out quite as much about the film's plot as we thought. The Avengers have a plan - but it's a lot more sophisticated than we believed.

More: Avengers 4 Theory Means Iron Man & Captain America Don't Reunite On-Screen

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