The full story
Natasha's origin story reveals her complex history. The film opens in 1995 Ohio, where young Natasha lives an apparently normal suburban life with her parents and little sister Yelena. Then the mission ends. Her "father" Alexei is a Soviet super-soldier. Her "mother" Melina is a scientist. The girls are assets, destined for the Red Room. The family was a cover operation.
The cover was fabricated, but genuine emotional bonds formed, which Natasha has suppressed for years.
After the events of Civil War, Natasha is a fugitive. The Avengers are fractured, the Sokovia Accords are in effect, and she is operating independently. Yelena—now grown, formerly in the Red Room—sends her a package: vials of a red gas that can break the chemical mind control keeping Widows enslaved.
The Red Room still exists. Dreykov, the man Natasha thought she killed years ago, is alive. He runs a global network of chemically controlled assassins, including Yelena until she was accidentally freed. Natasha's past actions have consequences she did not fully resolve.
To reach Dreykov, Natasha breaks Alexei—the Red Guardian, Russia's Captain America—out of prison. They find Melina at her farmhouse, still working for Dreykov. The family reunion is tense and emotional. These people care about each other despite their complicated history. Alexei seeks his daughters' approval. Melina is brilliant but compromised. Yelena is angry and hurt. Natasha wants to finish what she started.
The Red Room is a floating fortress in the sky, invisible and mobile. The final assault involves elaborate deception and Natasha confronting Dreykov. His control over the Widows has a fail-safe: they cannot hurt him if they can smell his pheromones. Natasha smashes her own face into a desk, severing the nerve.
Yelena kills Dreykov. The Widows are freed. Natasha sends them to deprogram their sisters across the globe.
The family separates again—Alexei and Melina to rebuild their lives, Yelena to find purpose. Natasha obtains a quinjet from Secretary Ross and heads to break Steve, Sam, and Wanda out of the Raft.
The post-credits scene, set after Endgame, shows Yelena at Natasha's grave. Valentina Allegra de Fontaine arrives with a new assignment: kill Clint Barton, the man "responsible" for Natasha's death.