In the MCU that we all know about, we saw how Steve Rogers transformed from a scrawny kid who hailed from Brooklyn to becoming Captain America. However, in an alternate reality, things did not go as planned, instead, it was Peggy Carter who got the super-soldier serum. So, how did she become the first Avenger in Marvel's What If..?
Warning! This portion may contain spoilers for Marvel's What If..?. Read at your own risk!
How did Peggy Carter Become the First Avenger Instead of Steve Rogers in Marvel's What If..?
In the pilot episode of Marvel's What If..?, titled What If... Captain Carter Were The First Avenger?, the canon show fully explained how Margaret "Peggy" Carter became the first Avenger instead of Steve Rogers, then having the title Captain Carter, donning a suit with British insignia all over and a shield that matches her outfit.
It was Steve Rogers, the original recipient, however, it could be remembered that in Captain America: The First Avenger, there were people from Hydra who wanted to take the serum and supposedly bombed the place had they been successful. In the film, they were immediately stopped and it was the first-ever chase of Captain America, the super-soldier.
However, things took a different turn in What If..? Episode 1, the bomb exploded just in time before the procedure on Steve Rogers went through. Being the scrawny big-hearted courageous man that he is, he got out of the machine but got shot in the process, rendering him unfit for the procedure of receiving the super-soldier serum.
When Howard Stark knew what has to be done, he declared that the serum has to be administered right then and there, or nothing at all. Peggy, knowing that there is no other option, jumped on the chamber without hesitation, and Stark started the process, therefore, creating a super-soldier.
Superiors didn't approve of it, of course. In the 40s, women were perceived differently, lesser than a man. She was even mocked she might lose a nail if she goes to the field. Still, Peggy proved everyone wrong when she stopped the convoy of Arnim Zola who handles the Tesseract. After that, she got her designation as a Captain.
She also single-handedly saved Bucky Barnes, the Howling Commandos, and the whole 107th infantry in the hands of Hydra. Steve got there as well, with Howard Stark creating a suit for him, the Hydra Stomper. After that, with her friends by her side, she was unstoppable, or so we all thought.
The familiar train scene was also there, but it wasn't Bucky who was taken – it was Steve inside the Hydra Stomper. Stark claimed it was an indestructible suit, which they later proved true when they set an attack on the castle where Hydra was experimenting with the Tesseract. Steve was there, along with the suit, and as for Hydra, they were taking a monster, a real hydra, out of a portal.
Peggy knew the risk. She had to call the shot. With a sword and a shield in her hands, she took one last look at Steve, setting a dance, a date, all these before pushing out the monster into the portal and costing her life as the portal closed. Seventy years later, the first Avenger, a new hero, came out of the portal and met Nicky Fury and Clint Barton.