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The Joke of Budapest.

Budapest had become a secret joke between Clint and Natasha. Secret because of who they were and what they did for a living, a joke because it had never actually happened. In fact, Budapest was one of the few places that neither Natasha nor Clint had ever been to. The first time Natasha brought up Budapest, during training in their main SHIELD base, Clint had glanced at her, confused. What on earth was she talking about? He wasn't sure whether she'd been or not at that point, but they'd certainly never gone together.

But then she'd sent him a subtle wink, and he'd decided to play along, just for fun.

And god was it fun. It confused several of the agents they knew more personally to no end, put the rookies in their places, and drove Coulson, Hill and Fury mad.

The second time Natasha brought up Budapest was on a mission, and Clint honestly hadn't been expecting it. She hadn't brought it up since that first time in the SHIELD base. He hadn't thought she would talk about it whilst on a mission. So when he heard Natasha suddenly murmur through the comms, "Does this remind you of Budapest too?" Clint raised his eyebrows at her, despite her not being able to see him.
"I'm not sure," he replied, "I don't remember the tracking lasting for this long. It was taking the marks out that took the most of the time."
"Then you and I remember it very differently," came Natasha's reply, "Tracking the marks took hours."
"If you say so," Clint said, almost indifferently, "It felt pretty quick to me."

Slowly but surely, Budapest somehow became their secret joke. They began referencing it whenever they felt like it. Clint particularly enjoyed the reactions of Fury, Coulson and Hill. Coulson quickly realised that it was some sort of joke between them and decided that he didn't want to know. When they brought it up, he merely rolled his eyes. Fury caught on fairly quickly too, and glared at them for bringing it up whenever they talked about it around him. Hill, knowing them the least well out of the three of them, didn't quite understand that it was a joke at first. She tried to ignore their talk of a Budapest mission that had never even existed, despite the near-constant pestering she got from the newbies and rookies about it whenever Fury made her look after them - an occurrence which never lasted for more than a day. But eventually, Hill's curiosity got the better of her, and she burst out,
"For God's sake! Neither of you have even been to Budapest! Why do you keep - oh, I see." It was then that she understood. She chuckled, grinning, to herself, and walked off. After that, she all but openly encouraged it. Maybe she figured that they deserved some sort of joke between themselves after everything they'd been through together.

Clint wasn't a bit surprised that Natasha brought up Budapest during the Battle of New York.
"This is just like Budapest all over again!" She called.
Ha, yeah, sure it is, Tasha, he thought.
"You and I remember Budapest very differently," he called back, echoing the words she had told him so long ago. He knew she would appreciate it. He also knew that their remarks would annoy Stark, a fact that would be a definite plus in Natasha's eyes.

Budapest was frequently referenced in their Avengers missions. It drove Stark mad. While Steve and Bruce mostly left them alone, Tony seemed to have no qualms about springing Budapest-themed questions on them at any time. He also seemed to have brought the ever-curious Thor onto his side, as Thor asked them about Budapest almost every time they brought it up.

"This is just like in Budapest!" Natasha called, one gun over Clint's shoulder as she took out several enemies. It was nearly two years after New York, and they were fighting HYDRA agents in Los Angeles.
"Er... yeah, sort of," Clint called back, grinning at the slight smirk on her face.
"For God's sake!" Tony yelled over the comms, along with a thump as he took out one of the enemies, "What the hell happened in Budapest?"
"Wouldn't you like to know?" Clint drawled, winking at Natasha, who, by the look of it, was seconds away from bursting into hysterical laughter.
"Come on, guys! Stop twisting our legs here!" he complained.
"Iron Man, focus," Steve interrupted.
"I am focusing," Tony replied, "But at the same time, I'm demanding some answers from our so-called teammates!"
"Uh oh, Stark's trying to multitask! Everyone take cover!" Natasha teased, as Clint chuckled. Together, they ducked both an enemy bullet and a blast from Tony as he flew by overhead.
"Thanks for that, Iron Man!" Clint called, shaking dust out of his hair.
"Anytime, Bird Boy," Tony replied, "So you were saying about Budapest?" Clint rolled his eyes at Natasha, who grinned.
"Let's just say that that's between Tasha and me," Clint grinned.

Their joke remained secret for another six months before Stark finally thought to look in their mission files. When Clint returned from a SHIELD mission from the resurrected Coulson with Natasha and they saw the look on his face, they knew that they were busted. The fun with the mystery Budapest mission for teasing the other Avengers was finally over.

"Hey, Tasha! This is just like Brittany all over again!" Clint yelled over the explosion of his arrow.
Natasha had sent a look over to him, but caught on to his game with ease.
"Almost, yes!" she called back, as she judo-flipped her opponent, smashing him to the floor behind her.
She knew as well as he did that neither of them had ever been to Brittany either, and Clint grinned when she got the hint.

They had a new game to play.