Clint Barton and Natasha Romanoff formed a partnership, and became some of S.H.I.E.L.D.'s top agents, as Barton was the one who recruited Romanoff into the agency. Throughout their partnership, they were sent on numerous missions together.
S.H.I.E.L.D. tasked Hawkeye and Black Widow to participate in a mission, as part of STRIKE Team: Delta, sending them to Budapest, Hungary. During the mission, they were involved in a situation that would later be remembered by Black Widow during the Battle of New York, as fighting against the Chitauri groups reminded her of the mission, something Hawkeye disagreed with. They were able to successfully complete the mission, not requiring an extraction plan.
Once they returned from Budapest, details of the mission were kept secret on S.H.I.E.L.D.'s files and the full details were only known by high-ranking members. However, in 2014 when HYDRA was discovered in S.H.I.E.L.D., Natasha Romanoff chose to release all of S.H.I.E.L.D.'s secrets on the internet to expose the terrorist organization, including details of her mission in Budapest. The mission would also go on to be incredibly memorable and defining in the agents' partnership, as Clint Barton told Romanoff, while they were flying to the planet Vormir in the Benatar after traveling through time, that they were a long way from Budapest.