From other thread:
ENTERTAINMENT WEEKLY: Is this an undead Spear story?

GENNDY TARTAKOVSKY: Yeah! He's a zombie. I'm not gonna talk about how he became one. We'll obviously reveal it in the show. I finished the [season 2] story and I was moving on. Then I started to have second thoughts. I just spent 20 episodes having people care, and then I kill off the main character, which is okay, but then, what are my options? The other stuff I was doing wasn't feeling quite right, maybe because I hadn't finished the story. And because we had that "Plague of Madness" episode in the first season where it was a zombie dinosaur, this is part of our world. It's pulp. It's the ultimate pulpy thing, the undead.

Are we still in the real world of Primal or are we in the underworld? There are so many images in that teaser trailer, especially the ending with a giant fly. Is this an otherworldly dimension?

No, no. What's great about it is it flows right to the next. So it's in the same world, same everything. There's dinosaur and man, then there's different civilizations, and then now there's this thing happening. I imagine this world has a lot of different parts, and some parts aren't discovered. Some people don't go into other parts because they know [not to]. So it's this f---ed up world that's really crazy, and now we've entered this part, but you feel like it's the same show. It's amped up and the story's crazy, but it feels like you're watching a Primal episode. It's not a reboot, it's none of those things.