The movie reunites Natasha Romanoff with her old "family," including Yelena Belova, Melina Vostokoff and Alexei Shostakov (Florence Pugh, Rachel Weisz and David Harbour). It also pits her against the deadly Taskmaster, a seemingly unstoppable killer who can mirror anyone's fighting style.
Before the explosions kick off on the floating fortress of big bad Dreykov (Ray Winstone), he reveals Taskmaster is his daughter Antonia, previously believed to have been killed in Natasha's failed attempt on Dreykov's life in Budapest, Hungary. Antonia (Olga Kurylenko) removes her mask to reveal her scarred face.
It turns out the little girl was badly injured in the explosion, so her father implanted her with a chip so she could instantly mirror anyone's fighting style and turned her into a brainwashed killer -- much like Yelena. Dreykov orders the other mind-controlled Widows to end Natasha, but she doses them with the antidote and frees them from his hold.
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Finding Antonia trapped in a cell as the floating fortress comes crashing down, a guilty Natasha opts to save her from certain death by letting her out. Antonia pursues her hero relentlessly as her programming demands, but Natasha ultimately manages to dose her with the antidote.
Meanwhile, Yelena almost sacrifices herself to kill Dreykov once and for all, forcing Natasha to save her as she plummets from the sky.
Safely back on the ground, Yelena, Melina and Alexei leave with Antonia and the freed Widows, intending to find the other brainwashed women around the world and free them. Natasha awaits the arrival of General Thaddeus Ross (William Hurt) and his team, apparently accepting being taken into custody for siding with Captain America's faction in Civil War.
Two weeks later
After a quick time jump, we find Natasha free. Her hair is dyed an Avengers: Infinity War-shade of blond and she's wearing Yelena's jacket (with all its handy pockets). She meets her buddy Mason (O-T Fagbenle), who proves how resourceful he is by supplying her with a Quinjet and asks where she's going.
"My whole life, I didn't think I had any family -- turns out I got two," she tells him. "So, one of them's a bit of a mess right now. Gonna go break a few of them out of prison, see if I can't help patch things up."
What it means
Melina and Alexei leave with Antonia and the freed Widows are still out there, apparently working to dismantle the remainder of Dreykov's operations. The post-credits scene reveals where Yelena ends up after Avengers: Endgame, but the others' fates are unknown -- their story could continue in a Black Widow followup of some kind.
Despite Natasha's final words to Mason, it's unclear if she actually participated in the operation to free Sam Wilson, Clint Barton, Wanda Maximoff and Scott Lang from the Raft prison. We only see Cap breaking them out at the end of Civil War, but Natasha is nowhere to be seen -- she could've been there, since we know she's teamed up with him by the time of Avengers: Infinity War.
We know Natasha's ultimate fate though -- she sacrifices her life to obtain the Soul Stone and undo Thanos' genocidal Snap in Avengers: Endgame. We see the fallout of her decision in this movie's post-credits scene.