The Fringe season 4 finale airs tomorrow night, and the promo revealed the return of Michael Cerveris as September the Observer! But how is he back?! Beware of spoilers.

Cerveris tells The Insider:

[September] does show up in Friday’s episode with some key bits of the puzzle to explain. But even he is caught off guard by some things that happen in the episode, which is nice because he’s normally a few universes ahead of everyone else. Some things happen that surprise even September – and will surprise the viewers for sure! [And] what happened to him after he disappeared from Walter’s lab is specifically addressed in the finale.

He also teases the finale in general, revealing:

The questions and the stakes that get raised in the finale are nothing short of the survival of reality and the survival of humanity as we know it. Satisfying in a sci-fi way, but also really satisfying in a philosophical way. And as always, with Fringe, able to be reduced down to something as simple as the relationship between a father, a son and the woman he loves. It opens the door to the future and this dystopian Observer-land. Which one would hope has room for little old September somehow.

But will this be September’s final return? It doesn’t seem like it! According to Cerveris, “As long as Fringe is on, I was told I’d be part of it.” Interesting.

“Brave New World, Part 1” airs tomorrow night at 9/8c on Fox!