There’s Enough History in This Room to Fill the Smithsonian
When the SSR is brought back to the Tower - which Steve said is called Avengers Tower, but Howard instantly recognized as Stark Tower, except almost entirely rebuilt - Peggy gets to witness something she never ever thought she’d see.
Howard Stark, standing next to a slightly shorter almost-copy of himself, who Steve introduces as Tony Stark, before things really get tense.
Tony, after all, is not at all hiding that he doesn’t want to be anywhere near Howard. Howard certainly isn’t improving matters, Peggy knows, by walking around the floor like he owns the place, questioning architecture design choices and more.
Tony’s face just gets tighter, like he’s been forced to swallow the world’s sourest lemon as is trying not to show it, when Howard starts inspecting the metal suit he steps out of.
Iron Man, they’re told, is the name of the suit. Peggy has never seen anything like it, and she knows Howard hasn’t either - but apparently, that makes him want to take it apart and learn how it works.
She’s positive this new Stark is going to explode.
It doesn’t take long to prove her theory right.
She doesn’t actually see the action which sets him off, but a mild-mannered man (once he stops being green and huge and, apparently, nearly indestructible) who introduces himself as Bruce Banner tells her later that Howard started fixating on the glowing device in Tony’s chest, which Bruce refers to as an “arc reactor,” which keeps Tony alive.
Also, he tells her, Howard turns out to be, unsurprisingly, a rather horrible father, from what little Bruce knows.
What Peggy sees is the immediate aftermath. Namely, Tony punching Howard in the jaw so hard, he probably fractures his hand, and likely Howard’s jaw. Howard certainly ends up with a black eye, and Tony looks like he’s ready to keep going, but the red-haired woman (who they’re later told is Natasha Romanov, a student of the Red Room they’ve come across in their own time) drags him out of the room.
A nurse who, they’re told, works for SHIELD, which is what the SSR will become, shows up and Howard goes willingly. Peggy gives a long-suffering sigh, wondering if he’s going to try to add women from the future to his long list of “conquests.”
She may find Tony and have him hit Howard again, if he does. She suspects he may deserve it.
As, apparently, they are not keeping their future a secret, the next two hours are spent introducing the Avengers to the SSR, giving a very rough explanation of how the SSR became SHIELD, with Peggy and Howard as founding members - one of her coworkers scoffs, remarking that a woman would never be the head of the SSR, no matter what its name, and Steve looks ready to murder him. Steve literally drags the man out of the room for a moment, and when they return, the SSR agent’s knees are literally shaking - and then abruptly, the now-metal-armed Bucky Barnes walks into the room, and Steve falls silent.
“How do you explain him?”
Steve and Bucky both glare at the SSR agent who spoke.
Peggy has to admit, the future-Bucky’s stare is almost enough to intimidate her. The poor agent is shaking harder than the one Steve talked to.
“Hail Hydra,” Bucky says, and it takes Peggy a moment to catch the bitterness in his flat tone.
All the other SSR agents tense.
That’s when the explanation becomes horrifying, to the SSR.
Peggy can’t believe that her life’s work - what Steve says her life’s work will be - amounts to scattered factions now, barely hanging on, because Hydra infected it in the beginning.
It’s only after several minutes of silence, the SSR in shock and the Avengers expressing sympathy with their silence, that Steve begins to ask them when exactly they’re from, and how they could have arrived in the future.
The latter, they don’t know, and neither do the Avengers.
The former, well.
That’s another issue.