The actor will also be reprising his role as Red Guardian in 'Thunderbolts' for Marvel.
David Harbour has not one, but two high-profile projects to keep under wraps.
The actor hilariously did his best to promote his roles in both Marvel's Thunderbolts and Netflix's Stranger Things at Disney's D23 Expo over the weekend, all while maintaining a level of secrecy both franchises have become known for.
Asked about what fans can expect from the upcoming fifth and final season of Stranger Things, Harbour quips to ET's Will Marfuggi, "Here's the storyline: so it starts with the death of..."
Trailing off with a laugh, he continues.
"Ah, no. You know, I mean, I know a little bit about where it's headed and it's really exciting, but you'll see it in another 15 years when it's able to be released when we've shot it."
Consider our calendars marked.
As for Thunderbolts, Harbour praises Marvel's casting choices -- which also includes the returns of Julia Louis-Dreyfus as Contessa Valentina Allegra de Fontaine and Florence Pugh as Yelena Belova, with Harbour as Red Guardian, Wyatt Russell as John Walker, Hannah John-Kamen as Ghost, Olga Kurylenko as Taskmaster, and Sebastian Stan as the Winter Soldier. Jake Schreier will direct the project, which is currently in pre-production.
"There's a lot of really exciting work happening on this movie," Harbour says. "I also think it's a unique MCU movie in the sense that you've got a bunch of misfits and outcasts and losers and people who don't really live up to the super in superhero. And also all of us are such cool, interesting performers -- I think audiences have some complicated feelings about a lot of us -- and I think that's a terrific thing going in."
ET also caught up with Louis-Dreyfus on the red carpet, where she joked that her role in Thunderbolts "means that my children now really respect what I do for a living,"
See her interview below.