WARNING: The following contains spoilers for What If...? Season 1, Episode 1, 'What If... Captain Carter Were The First Avenger,' streaming now on Disney+.
After Loki's finale opened the door to the Marvel Cinematic Universe's Multiverse, What If...? is here to explore the endless possibilities that branched from the Sacred Timeline. The first episode is 'What If... Captain Carter Were The First Avenger,' and it explores a reality in which Peggy Carter was the one to take the super-soldier serum, rather than Steve Rogers.
'What If... Captain Carter Were The First Avenger' opens with narration from Jeffrey Wright's the Watcher, who explains that "a single choice can branch out into infinite realities" before introducing the audience to the episode's premise. The show then cuts to the moment in Captain America: The First Avenger in which Steve is preparing to undergo the process that gives him the serum. However, in this reality, instead of opting to stay in the booth to watch the procedure, Peggy decides she'd rather stay downstairs.
Events play out differently from that point on, with the Hydra assassin setting off his bomb before Steve's transformation is completed. Wanting to help, Steve rushes into action but is quickly shot, just as Peggy rushes in and kills the man. With the program's only recruit unable to finish the process, however, Peggy decides to take the serum herself, left they lost all chances of anyone ever getting it.
As was all too common in the '40s, the man in charge of the Strategic Scientific Reserve, Colonel Flynn, isn't pleased about having a female super-soldier and promptly benches Peggy. Of course, despite the changes to Steve and Peggy's stories, the MCU still has a villain on the loose in the form of the Red Skull, who is looking for the Tesseract. He finds it in Norway, and while Peggy and Howard Stark believe they can intercept it before any real damage is done, Flynn shuts them down.
Regardless, Howard gives Peggy her USO outfit and a shield, and she decides to go after the Tesseract herself. Transporting the object alongside plenty of Hydra goons is Armin Zola, who Peggy takes prisoner after making quick work of his colleagues. She returns to Flynn and tells him she'll be needing a promotion to captain before handing over the Tesseract to Howard. And with the power of an Infinity Stone in his hands, he builds a suit reminiscent of Tony Stark's Mark 1 suit so Steve can join the war effort.
In need of allies to fight back against Hydra, the newly dubbed Captain Carter decides to rescue the 107th, a division that includes Bucky Barnes, Steve's lifelong friend. She enters the facility with ease and rescues the future members of the Howling Commandos and Bucky, who has yet to be subjected to Hydra experimentation. However, the group encounters some resistance on their way out, which is the perfect opportunity for Steve to swoop in and use the "Hydra Stomper."
Before long, Captain Carter turns the tide of the war and puts the SSR closer than ever to putting an end to Hydra's plans. But Red Skull has something new up his sleeve: without access to the Tesseract, he decides to seek out a champion from the stars to help defeat Captain Carter. The SSR then gets word that the Red Skull is on a train, so Captain Carter and Co. attempt to seize the opportunity to end the war. However, the whole thing is a trap set up by Hydra, and Steve is presumed dead after the train explodes.
Upset, Captain Carter interrogates Zola, who tells her of the Red Skull's plan to unleash an interdimensional creature on the world and gives up Hydra's location. The forces of the SSR, including Howard, then head to Hydra's headquarters, where they split into two teams. But just as they arrive, it's revealed Hydra was able to require the Tesseract after the train incident and is using it to open a portal to another part of the universe. Captain Carter heard the commotion, and soon, she and Howard come face-to-face with a giant creature, which quickly kills the Red Skull.
The second team, led by Bucky, finds Steve in the basement, and after a brief reunion, he fires up the Hydra Stomper to help Peggy in her fight. Back upstairs, Howard devises a plan to reverse the polarity of the portal to suck the creature back in, but he needs time to figure out the controls. With the creature threatening to bring the whole facility down, Steve gets the Howling Commandos to safety but returns to find Peggy forcing the creature back into the portal using her brute strength. She walks into the portal as she tells Steve about the dance he owes her, mirroring the iconic moment from The First Avenger.
70 years later, Captain Carter returns and walks through the same portal Loki did in 2012's The Avengers. Nick Fury and Hawkeye greet her, with the spy informing the hero of how long it's been and how the war ended.
To see Captain Carter's adventures, the first episode of Marvel's What If...? is streaming now on Disney+.