If I Burn (Avengers/Agent Carter Crossover)

"Their planet had gone up in flames, charred beyond recognition. All of the buildings were gone. They were now tangles of steel that the remaining animals used to hide. Maybe, if Natasha was optimistic, there were people hiding down there as well. But she doubted they had any sort of humanity left. They weren't human anymore. None of them were. Earth's Mightiest Heroes had failed everyone."

A post-apocalyptic au where Ultron was actually a major threat and defeated the Avengers. Due to this, Tony decides to take matters in his own hands with the remaining technology they have left. He wanted to save his teammates, so he sent them to the safest place possible. 1946. That's where Peggy Carter came in.