There'd been a lot of chatter about a major arms dealer named Nicholas Nazari showing his face around Chicago. SHIELD had been monitoring the situation; an anonymous tip had been verified that an important meeting was to be made with his unknown supplier. SHIELD wanted to apprehend this unknown supplier and take down all of their connections; causing huge disruptions in the illegal weapons trade.
That's where Agents Romanoff and Barton came in.
Their cover story had been simple enough; newlywed couple Aaron and Rosie Watson had dropped out of college when she'd been diagnosed with a brain tumour. Three years later, and now fully recovered, the two had gotten part-time jobs in the town center (right by Nazari's favourite cafe where the meeting was supposedly taking place) in order to pay for night school to finish off their qualifications (teaching and pre-med).
Natasha hadn't been impressed to learn that her undercover assignment included working in a jewelers; but felt instantly better on learning Clint's cover story required him to work at the hot dog stall and involved him wearing an apron and a cap. She'd laughed so hard when she first saw him in his uniform! At least Natasha just had to wear a suit and a name tag.
The first few days passed uneventfully; they'd planted some cameras around the busy town center for SHIELD to examine, their status as a married couple allowed them to spend their lunch breaks together without fear of being seen, and they monitored the busy streets as best they could; looking out for anything unusual or any people frequenting the area.
It was on the fifth day of their undercover work that things went drastically wrong.