In her own Marvel stand-alone picture, the title hero, an ex-KGB assassin, springs into action after the events of “Captain America: Civil War.”

“I used to have nothing,” the title character, played by Scarlett Johansson, says in the first trailer for “Black Widow.” And “then I got this job, this family, but nothing lasts forever.”

Nothing except, it seems, the Marvel Cinematic Universe. The first entry in the series since the apparent finale,