And here's how it all clicks together - at least, in my twisted little mind. :D

The Bourne Connection

Barton must have talked to Fury right away, for the very same evening, he gathered them all in the living area.

"I guess Steve's told you all what happened earlier today", he said.

Steve nodded, and so did the others.

"Right."
Barton took in a deep breath.
"Fact is – yes, I am Aaron Cross."

"Hah!"
Steve blushed when everyone suddenly was looking at him.
"Sorry, it's just – I expected this."
He gestured at Barton to continue.

"All of this happened back in 2005", the archer went on as if he hadn't been interrupted. "It was my first big undercover mission for SHIELD. I mean – really big. I don't know if you caught any of it but at that time, there were several black ops programs run by – I'm not really sure, actually, Fury never told me. But the CIA might have been involved. Wouldn't be surprised if they were. Anyways, one of them was called Treadstone. A guy named Jason Bourne caused them a lot of trouble. The one SHIELD had got me into was called Outcome. It was –"

He glanced at Steve.

"Well, let's say some kind of supersoldier program. They used different stems of viruses and genetic engineering to enhance people both physically and mentally. SHIELD of course wanted to know everything about it. However, due to that Bourne-guy, the folks who ran the program decided to shut it down."

"Let me guess", Tony said. "'Shut down' didn't mean 'tell everybody goodbye and send them home', right?"

Barton nodded.
"Right. In this case, they decided the best thing to do would be to kill all the test subjects and the involved scientists. I got away – and so did that woman we met in the street today."
He looked at Steve.
"Her name's Marta. Dr. Marta Shearing. Or rather, it used to be. I don't think she still goes by that name."

"She said she thought you were dead", Steve pointed out.

"Yes. As I explained, we were the only two survivors from that project – except for the people who were behind it all, obviously. Of course, I could have gone back to SHIELD. But I wanted to bring them samples of the meds they were giving us. I didn't know, then, that they were actually using viruses to – well, implant us with DNA to enhance us. That's what Marta explained to me."

Barton paused a moment, probably to see if they were still following.

Steve wasn't really. That talk about viruses and DNA kinda went over his head. But he got the supersoldier comparison. Basically, that project had tried to make people stronger and smarter and obviously, it had been a success.

"Basically, there were two kinds of pills: green ones for the physical part and blue ones for the mental part", Barton went on. "After they'd tried to kill me, I knew I had to get out of there. I wanted to bring samples of both to SHIELD but had run out of the green pills and was running out of blue ones, too. So my best bet was to find the one surviving scientist and get her to give me more of that stuff. Well, turned out she didn't have any. But she told me that they had already injected me with the virus to make the physical enhancements permanent. And that we could find the other virus in Manila. So that's where we went."

He rubbed his neck.

"I'm not gonna bore you with the details. We managed to get our hands on that second virus, she injected me, I got so sick I thought I'd die but we still managed to get away from the killers they'd sent after us. After that, I set her up with a new identity and then, arranged for SHIELD to 'catch' me so she'd think I were dead and move on with her new life."

Again, Barton stopped and looked at them.

"So you're what?" Tony asked. "A super-smartass with super-strength?"

The archer shook his head.
"I was for a while. Not anymore, though."

"Something went wrong", Steve guessed.

"Yes. That 'blue' virus was still – let's say, a prototype. They hadn't managed to get all the kinks out. I returned to SHIELD, their scientists took my blood – bloody vampires, all of them – and tried to figure out what exactly those other blokes had done. Then, things started to go wrong. Only slight headaches at first but they soon got worse."

Barton shrugged.

"Turned out that 'blue' virus was out of control and changing stuff it wasn't supposed to change. Luckily, SHIELD's got some pretty darn amazing scientists on their payroll or else, I wouldn't be here now. They figured out what was going on and found a way to counter it. Unfortunately, it also countered the effects of the 'green' virus, so most of the enhancement's gone now."

He looked at Tony.
"Or in your words, I'm just a little smarter and a little stronger than I used to be. Enough to make me one of SHIELD's finest, though."
He grinned at that.

Steve needed a few moments to let this story sink in. So did the others, obviously, for they remained silent, too.

"Will you try to find her and tell her you're still around and kicking?" Tony finally asked.

"Who? Marta?"
Barton shook his head.
"No. It's better that she keeps thinking Aaron Cross is dead. He is, after all. Well, kind of."

Tony opened his mouth, probably to ask another question but he didn't get to it.

"You've been cheating!" Tasha suddenly burst out.

Her glare would have made everyone who didn't know her back away in terror.

Barton simply frowned.
"Cheating?"

"Yes. In training. You've been cheating."
Tasha closed the distance between them and stabbed her index finger at him accusingly.
"You could only keep up with me the way you did because of those enhancements."
She frowned, too.
"In fact, you could only catch me –"

"Because Coulson came up with a cunning plan to catch a deadly spider", Barton interrupted her. "That one wasn't actually my idea, you know?"

"Still – in training, you cheated."
Tasha folded her arms across her chest.

"Fine, fine. I cheated. Sorry for that."

Barton gave her an exasperated look.
"If that's all you're worried about –"

"No", Tasha said. "But I guess SHIELD's science department got it all under control or else, you wouldn't be allowed out in the field."

"Well, since they give me a complete and thorough check-up every six months, they better have it all under control cos if they don't, I really don't wanna know what they need all that blood for."

"You said it yourself."
Tony shrugged.
"Vampires."

Barton stared at him. Then, he started to laugh. The others, including Steve, joined him.

The archer grew serious again.

"Right, people, I didn't only tell you all this cos I knew Tony wouldn't rest until he had found out everything about Aaron Cross", he said. "You know the story now. If something should go wrong – with me, I mean – go and find Marta. She knows more about this whole shit than anyone else. SHIELD's got great scientists and all but Marta's the one with the inside knowledge."

"Who would we look for?" Steve asked.

Barton rubbed his neck.
"Well, she might have changed her name again but you might want to start with June Monroe and go from there."

June Monroe. Steve repeated the name in his mind several times so he'd remember it. Of course, he hoped he wouldn't need to but you never knew.