When Black Widow was first introduced to the Marvel Cinematic Universe in 2010's Iron Man, fans instantly wanted to know more about the badass S.H.I.E.L.D. agent sent to monitor Tony Stark. But the time 2012's The Avengers rolled around, the enigma surrounding her had grown, especially once viewers learned she and Hawkeye, who had first appeared in 2011's Thor, had served together on several previous missions. The most intriguing was Budapest, which they compared to the Battle of New York. Nearly a decade later, Black Widow finally revealed Clint and Natasha's Budapest mission and why it was so important.
Warning: Spoilers for Black Widow follow. Black Widow didn't immediately bring up Budapest in the film's opening moments, but even before it premiered, fans knew the film would return to the fateful city where Hawkeye and Black Widow teamed up. That's because footage in the trailers clearly showed Natasha in Budapest, coming full circle with her past.
But what might have startled fans is why Natasha went back: not for Clint, but for Yelena, who had taken refuge at a safe house in the city. As fans discovered, it's the same one where Natasha once resided. It turns out, that's because Natasha's little sister was on the run from the same people this time: Dreykov and his minions, the Black Widows.
The last time Natasha had been in the city, she’d also gone up against Dreykov, the mastermind behind Russia’s original Black Widow program and the new, improved version. Natasha had been part of the former team; she’d been emotionally manipulated into serving her motherland. Yelena was part of the new generation — one in which the program used chemicals to physically force the girls to do Dreykov’s bidding, despite their minds rebelling. Freed from her chemical-induced haze early in the film, Yelena had vials of the antidote and wanted Natasha to help her spread it.
But Natasha didn’t believe Yelena that Dreykov was behind the new program, and for a good reason: Natasha had already killed him. That was the Budapest mission and her ticket out of the Russian program and into S.H.I.E.L.D. She had to kill Dreykov, end his operation, and prove her loyalty once and for all. She and Clint took Dreykov out — blowing up the building in which he resided, along with his young daughter, killing them both.
Natasha still felt guilty for killing the child. (And at least some people know about it; Loki notably taunted her about “Dreykov’s daughter” in The Avengers.) But she felt she had no choice and no time to second guess either. Once she did the deed, she and Hawkeye spent weeks hiding inside the subway HVAC system, waiting out all those ready to enact revenge.
Natasha never knew that despite how hellacious it was, the Budapest mission wasn’t as successful as she’d believed. Dreykov survived. It may have been her ticket out and one of the worst battles she’d ever found herself in. But now, years later, she discovered it was time to finish the job.