Marvel fans have been given practically no time at all to recover from the massive, multiversal events featured in Spider-Man: No Way Home. On Super Bowl Sunday, they reminded us once again that another big movie is just around the metaphorical corner. The second trailer for Doctor Strange in the Multiverse of Madness was released along with a short TV teaser and an official poster, all noting the film's scheduled release on May 6.

But the release date isn't the only thing people are finding on the poster. The poster is riddled with little potential clues about the plot of the movie, including one major hint at a surprising character who may show up. If you zoom in on just the right piece of broken glass, you can see the reflection of the shield belonging to Captain Peggy Carter (Hayley Atwell) in What If...?.

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For the casual viewer, an appearance from Carter would be surprising. Even for many diehards, it could get confusing. Would Atwell's return in Multiverse of Madness retcon the ending of Avengers: Endgame? Most likely not! But how she could fit in the narrative alongside Doctor Strange (Benedict Cumberbatch) and Wanda Maximoff (Elizabeth Olsen) is currently unclear. So let's break down Peggy's history in the Marvel Cinematic Universe and why it wouldn't ruin anything if she showed up in Multiverse of Madness.

The character first appeared alongside Steve Rogers (Chris Evans) and Colonel Chester Phillips (Tommy Lee Jones) in Captain America: The First Avenger. She was a key part of the Strategic Scientific Reserve (SSR) during the war and continued to take down HYDRA in that role after the assumed death of Captain America.

Agent Carter ran for two seasons and 18 episodes on ABC, cataloging Peggy's life with the SSR in the short period following World War II. She spent some time working alongside Edwin Jarvis (James Darcy), who was briefly seen in Endgame, and went on to be one of the founders of S.H.I.E.L.D. along with Howard Stark (Dominic Cooper). The show was popular among its target demographic, but it ultimately wasn't enough for the network to renew Agent Carter for a third season.

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Although we know that Carter spent the rest of her career in service of S.H.I.E.L.D., that is mostly off-screen information. Steve steps into Peggy's office and sees her through a window briefly during Endgame, and this moment takes place in 1970. She's seen again in the opening scene of Ant-Man alongside the elder Stark, which happens in 1989.

In Captain America: The Winter Soldier, we learn that Carter is still alive but clearly not well, as she is seen in an assisted-living facility and would be nearly 100 years old. She is completely lucid in her conversation with Steve early on but abruptly forgets how he got there and that he was frozen in the ice. Although she is unseen in Captain America: Civil War, Rogers learns of her death from her niece, fellow S.H.I.E.L.D. agent Sharon Carter (Emily VanCamp), and Peggy's funeral is a major turning point for Rogers in the movie.

Endgame brings Carter back around for one last tug on the heartstrings, revealing with the film's final scene that Rogers chose to stay in the past and live out a happy life with her. Marvel has been tight-lipped about where the story goes for Steve and Peggy after this moment, even teasing fans in The Falcon & the Winter Soldier that there is a rumor Rogers is alive and living on the moon after the events of Endgame.

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Image via Disney/Marvel

But if there's one thing of which we can be certain, it's that Atwell's potential appearance in Multiverse of Madness won't change anything about the happy ending Steve and Peggy enjoyed together after Endgame.

That slow dance with Steve was not the last time we saw her, and that's where things get interesting. Atwell reprised her role as Peggy Carter in Marvel's animated Disney+ series, What If...?. The premise of the show was to present the viewer with an anthology, of sorts, showing how familiar situations from the MCU happened differently in other universes.

In the debut episode, a key decision by Carter during the experiment on Rogers results in a completely new timeline where Steve doesn't get the super-soldier serum. In this separate universe, it's Carter who becomes the iconic hero. She's outfitted with her own suit and a shield bearing the British flag in the center. Stark creates the HYDRA Stomper suit for Rogers, which is visually similar to the very first, crude suit created by Tony Stark (Robert Downey Jr.) in Iron Man.

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Image via Marvel Studios

Much of the remaining plot of Carter's What If...? episode plays out similarly to The First Avenger, except notably with Steve being lost during the train scene instead of Bucky Barnes (Sebastian Stan). The story ends with Carter sacrificing everything to save those around her, much like Rogers did in the main Marvel timeline.

The final entry in the first season of What If...? brings the key figures from each episode together for a battle against an Infinity stone-wielding Ultron (Ross Marquand). Carter teams up with variants of T'Challa (Chadwick Boseman), Thor (Chris Hemsworth), Doctor Strange (Benedict Cumberbatch), and others to contain this threat against the multiverse.

And now, it appears she might be crossing over with Strange once again. Both of the trailers that have been released for Multiverse of Madness contain imagery that is reminiscent of What If...?, such as the inky look to the disintegrating buildings or the darker, scarier version of Strange. With rumors of possible cameos and the promise that things are going to get super weird, bringing back Peggy Carter didn't feel like such a stretch even before the movie poster clue.

What role she might play in the movie is anybody's guess, at this point, and she may be reduced to a cameo. Maybe her appearance will just be a shout-out to the fans of What If...?. However, there is a segment of fans who enjoyed Agent Carter and were unhappy when the show was canceled after its second season. Atwell has expressed an interest in returning to the live-action MCU in the past, and the multiverse allows for her to rejoin the fray without too much painful exposition needed to explain it.