Huh. At Amazon, the DVD is more expensive than the Blu-ray. Well, it showed up for no (extra) fee at Disney+, and I have no interest in a Red Sox-free World Series, so…
It's the origin story of Natasha Romanoff. (I'm not sure if the Marvel movies have ever explicitly called her the "Black Widow" before.) It's pretty dark: girls plucked off the streets or taken from their parents by Soviet Union baddie, Dreykov. Only the ablest survive to go on missions of espionage/sabotage/assassination. A pre-teen Natasha gets implanted in Ohio with her phony "family": dad Alexei, who (somehow) got the super-soldier serum to become the "Red Guardian"; mom Melania, the brains of the cell, and little sister Yelena, (who thinks the"family" is real. Sorry, kid.
After completing the mission, their cover is blown, and they make a harrowing escape to Cuba, reality is revealed to little Yelena, just as she's whisked off for spy training and girl-parts removal. Flash forward to a point in the MCU timeline just around the time of Captain America: Civil War; Natasha is on the run for being on the wrong side. But she gets word that that old baddie Dreykov is still around; way back in pre-Iron Man 2 days, she thought she had killed him in order to win the favor of SHIELD.
So she teams up (after some sisterly-but-really-not fighting) with now-grown Yelena to track down the still-dangerous Dreykov and his deadly army of mind-controlled Widows. Again, involving explosions and lots of CGI.
It's all pretty neat, and the post-credit scene makes me want to see what happens next.