Natasha sacrifices herself in the middle of a mission to keep Clint safe. Now that he's recovered, he's trying to get her back.

His vision was blurry. He could make out the hazy form of Natasha standing with her back to him. Shadows danced around in the corner of his eyes. He was worried about them. He feared them. He wanted to fight them. But he couldn't get up. Pain radiated from his stomach.
Natasha's voice came from very far away. "If I go with you, will you promise to let him go?" No, he tried to say. Nat, no. His tongue was too thick to speak.
A wave of murmurs washed over him, and he almost lost consciousness trying to distinguish them. When he opened his eyes again, Natasha was crouched in front of him. "I called Agent Hill. She's coming to get you."
"Nat," he protested weakly. Natasha shook her head and put her hand on his cheek.
"Hey, it's ok," she soothed, but there was uncertainty in her grey-blue eyes. "It's going to be ok." A hand appeared on her shoulder and dragged her back. Clint struggled to stand, to lift his bow, to do something to help her as her hands were tied behind her back and she was forced to her knees. His eyes drifted shut as he heard that unmistakable sound of a gun hitting flesh and flesh hitting ground.
Clint sat up in bed, gasping for breath. He looked around the dark hotel room, trying to convince himself that it wasn't real, that it was only a dream. His breathing slowed, and he looked to his right as if searching for Natasha. His heart plummeted as the empty space triggered vivid memories of the last month.
She was gone, and it was his fault.
He had to get her back.
Realizing that sleep was impossible, he threw the covers off and went to the balcony, frowning at the glittering city skyline. It wasn't long ago that he had held her in his arms, looking at the same city from the top of the Avengers Tower. It was the night before everything went wrong. They were just finishing up their week-long vacation when a SHIELD agent contacted them with the mission. It was nothing more than a drugs bust, they said. Shouldn't last more than a day. But when they burst into the warehouse, it was a terrorist group that greeted them with open arms and machine guns.
And where did that leave them? The traitorous agent was imprisoned and facing charges for treason, Clint gained a gut wound and three weeks of bed rest, and Natasha was nowhere to be found.
Tony had spent the past several days meticulously combing New York for some sort of clue as to where they had taken her, with no such luck. Now that Clint had finally been released as fit for duty, he assumed full responsibility for the mission.
I will find you, he promised the not-quite-silent night. And I will tear them apart.