The Marvel universe is getting turned Upside Down.

"Stranger Things" star David Harbour is joining the cast of "Black Widow," the standalone movie starring Scarlett Johansson as the Avengers heroine, according to the Hollywood Reporter.

Harbour's casting comes just a few weeks after Florence Pugh hopped on board the movie. Details of who either will be playing have not been revealed.

Not much is known about the plot, other than that it will be a globe-trotting story featuring Natasha Romanoff, the spy who was trained by the KGB before becoming an agent of SHIELD and an Avenger.  

Harbour, a veteran actor who gained leading man status on Netflix's supernatural drama series "Stranger Things,"  is headlining his first film, a reboot of "Hellboy," opening April 12. "Stranger Things" Season 3 premieres in July.