Walter Bishop takes matters into his own hands, in this totally horrifying moment from last night's episode of Fringe. We've been told a few times that Walter is losing his marbles, but this absolutely terrifying scene proves that actions speak louder than words.

In a world of alternate universes and murderous symbiotic fungi, how do you even define insanity? What counts as a grip on reality anyway? Spoilers ahead...

In the final scene of last night's "Alone in the World," Walter has just managed to kill a small boy's semi-imaginary friend — a fungal organism that homed in young Aaron's loneliness and formed a tight emotional and physiological bond with the boy, killing Aaron's enemies as well as a few other random people. Walter figures out how to make Aaron feel secure enough emotionally to loosen his bond with the fungus, allowing Olivia and Lincoln to kill the fungus' actual physical body.

And now, having done away with the unhealthy friend of a young boy who's lonely and isolated, Walter's determined to do the same to his own imaginary companion: the man whose face and voice he keeps hallucinating. Walter chooses to do this by lobotomizing himself with a railroad spike — the same solution he considered for the boy. (Or maybe he just intends to kill himself, it's really hard to tell from the texts he's looking at. I don't think a lobotomy would go through the eye socket like that. Update: Various commenters have pointed out that lobotomies do, in fact, go through the eye socket.)

It's a particularly horrifying moment, even for an episode where people are killed by deadly mold that grows all over their bodies and causes them to decompose rapidly and then explode. (The biggest mystery in the episode? Why the agents wear hazmat suits around the fungus once, and then randomly decide never to wear hazmat suits again. Maybe it's a fashion choice.)

The main plot of the episode, with the deadly fungus named Gus and its only friend, was sort of entertaining but instantly forgettable — mostly, it's just there to illuminate what's going on with Walter in the episode. And thank goodness we finally got an episode focusing on Walter — the first two episodes of the season felt awfully lacking in our favorite mad scientist. Thank goodness "Alone" redressed the balance.

Besides the final attempted lobotomy, the episode gave us plenty of other signs of just how disturbed and messed up Walter is, in a universe without Peter. The terms of Walter's release from St. Claire's Hospital are a lot less generous in this universe, and he has to have monthly visits with Dr. Sumner, who keeps him heavily medicated.

Walter tries to pretend that everything is fine, because he doesn't want to be re-committed, but he's a lousy liar. And the aural and visual hallucinations get so bad at one point, they distract him when he's trying to sound professional talking to Agent Broyles. The way he yelps, "I am not losing my mind" at Broyles is pretty intense, and John Noble does an amazing job of conveying that Walter is on the edge of the abyss.

And we find out what we already suspected — that in this version of reality, Walter still crossed universes to save the other Peter. But in this version, he failed, and young Peter drowned in the lake instead of being rescued by the Observer. Walter's still totally raw and in pain over watching Peter die twice, so that he freaks out when Aaron picks up one of Peter's old toys.

When we see the fun, playful side of Walter, it only accentuates the fact that this is a much more precarious version of Dr. Bishop. He's still pretty hilarious, asking for a grape popsicle once he's done bathing Aaron in ice, and making his own chocolate milkshakes. And throwing out crazy speculations about the cause of the two boys' condition, including an attack by Bigfoot. And Walter's assessment of the Toy Story franchise is also pretty amazing.

As Walter opens up and bonds with Aaron, he shows a gentle, human side of him that we haven't really seen otherwise this season, and he seems to be finding a replacement for Peter — and perhaps it's not a coincidence that he stops seeing Peter around this time. Walter identifies with Aaron's isolation and his need for magic mushrooms, and then he has to watch Aaron nearly die as the Fringe crew attempts to kill Gus.

Oh, and this is our first mention of Nina Sharp and Massive Dynamic this season, I believe. They supply the toxin to kill the fungus — but apparently Walter no longer owns the company in this reality. Poor Walter. Easy come, easy go.

In the end, Walter saves Aaron by opening up all the way and saying that he, too, knows what it is to be lonely. And how hard it is to have the courage to reach out and take someone else's hand. Walter makes all sorts of promises he'll never be able to keep, like that he'll never leave Aaron and that he'll be there for the boy. Walter's emotional openness is somehow enough to get Aaron to release his fungal security blanket, so that Gus is able to die at last. (The whole "convincing the boy to let go of his symbiotic fungus" thing reminded me a lot of the "Olivia convincing a widow to let go of her alternate-universe dead husband" thing last season — neither scene entirely works for me, for similar reasons.)

So in the end, Olivia comes in just in time to stop Walter from auto-trepanation. And luckily, it turns out that Olivia's been seeing Peter too — only, in her case, it's in her dreams. (We get one brief moment foreshadowing this revelation, when Olivia's looking through a facial recognition database trying to match the face she's been seeing.)

Olivia shows Walter her sketch of Peter, and Walter recognizes that it's the same man. (Although, how does Olivia know that he's not just deciding retroactively that it's the same face, since he's just admitted he's going insane?) And now that they both know that they're seeing the same person, who apparently does not exist, they can start trying to figure out what's going on. Which means the "Where is Peter Bishop" storyline can finally shift into high gear. Here's hoping, anyway!