We've been wondering for three years if Gemma was actually alive or not, if Ms. Casey was a clone, etc, and we know definitively now that not only is Gemma alive, but that Lumon has been holding her prisoner while essentially torturing multiple other severed identities of her, each based on her fears/history with Mark.
That's major, major information, the likes of which we've never been given before.
We also got all kinds of behind-the-scenes teases at what Lumon is up to, which has helped further advance multiple theories as well.
That said, it's not all about plot answers.
For instance, before this episode, I - and even Damon Lindelof, re: the video I posted - was convinced that this show, at its heart, was going to be about the Mark/Helly/Hellena relationship, that Ms. Casey was likely a clone of some sort, and thus the Mark/Gemma (relation)ship had sailed. Mark would of course fight to save Gemma, but ultimately he'd end up with Helly/Hellena, let go of his past, his grief, and thus Gemma, etc.
But now?
This episode changed everything in that regard. And like I said earlier, I now not only have a seething hatred for Lumon I didn't have prior, but a new-found empathy for Gemma I didn't have prior, and no f-ing clue where this love triangle (sextuple?) will end up.
As invested as I was in this show prior to last night, it's nothing compared to my investment generated now.