Small, slightly shippy if you chose to read it that way drabble inspired from two lines of dialogue from Agents of SHIELD s01e07 -

"Barton, Romanoff, they never have an extraction plan" - Hand

"They know that going in" - Coulson

They never have an extraction plan, no matter how simple or complex a mission they are assigned. They know that there is always a good chance that one or both of them won't be coming home. It's a fact of their lives as SHIELD agents, but one they have acknowledged just three times over the course of their long partnership.

The first time was before their first mission. The then new partners agreed that if one of them were to be killed or captured the other would complete the mission, and if in the event the captured agent could not be recovered or had been killed, the one who lived would move on with their life.

The second was many many years later, after their assignment in Budapest went haywire. The resultant fight was considered so epic that it was whispered about for months not only amongst the helicarrier crew who witnessed it, but various versions spread far and wide through the whole of SHIELD's operations. It was no surprise to anyone that immediately following the fight the two incensed agents approached Coulson separately, requesting long-term solo assignments FAR away from each other.

The third time was the aftermath of the Battle of New York late that first night as Natasha held tightly to Clint, her usually stoic partner shuddering and gasping for air, the bodies of the innocent people he had killed under Loki's influence flashing through his nightmares. The gentle words whispered in bed that night were a balm to both partners hearts after the events of the past week, and nearly a year's separation and stubborn silence.

They still have no extraction plan going into a mission, but now they know that they will either live or die together, because living without the other is simply intolerable.