Fringe's Walter Bishop has long been one of the most fascinating characters on television, but his contradictions have seldom been as sharply portrayed as they were in last night's "Subject 9." This newest version of Walter might be the closest we've come yet to seeing the ugly truth about our favorite mad scientist: He's responsible for incalculable suffering, and he's incapable of coping with the world. The result made for mesmerizing viewing.

Spoilers ahead...

The above scene, in which Walter trashes a hotel room because of the deadly germs and assorted revolting stains he's convinced lurk everywhere, is like a dark reflection of the famous "Strawberry-flavored death" scene from season two. Walter completely loses control over his mind for a moment, to the point where he accidentally slices open both hands — and he stands there, in shock, holding out his two stigmata with a look of total horror on his face. Then, as Olivia bandages his hands, he opens up a bit and talks about his late wife, who committed suicide after their child Peter died.

So is Walter a Jesus figure, suffering for our sins? Not exactly. It's more like he's suffering for his own. This episode managed to throw two different facets of Walter into sharp relief: the fact that he's deeply and irreparably damaged, and the fact that he did unforgivable things to the Cortexiphan children. (And the episode doesn't even get into the whole "crossing universes and wreaking untold havoc" thing.)

Without the consolation of Peter in his life, this Walter has made much less progress, and his relationships are much more poisonous. He seems to have an okay relationship with Olivia, even though she can fully remember the Cortexiphan trials and the fact that she set a room on fire with her mind. But his relationship with Nina Sharp is so poisonous, he can't keep himself from spitting invective when Olivia and Astrid are visiting her at Massive Dynamic.

(Olivia, meanwhile, has a surprisingly close relationship with Nina Sharp, who seems to have known her since she was a kid. Not only did Olivia kill her stepfather in this reality, she also ran away from the Cortexiphan trials, and formed some bond with Nina Sharp.)

This was the first truly great episode of Fringe season four, and one which gave me a lot of hope. The show finally refocused back onto the elements that made it essential viewing in the past, in particular the tangled legacy of Walter Bishop. At last, we got to see Walter outside his lab once again — and the revolting, disorienting fresh air might not have been good for him, but it did wonders for our ability to appreciate him.

Walter's decision to leave the lab doesn't come from Walter's feeling comfortable and secure in himself — if anything, the episode puts Walter under immense pressure almost from the very first scene. Walter got a sort of reprieve at the end of last week's episode, as he discovered that the imaginary man who was appearing to him in the lab wasn't really imaginary. Or at least, that Olivia seemed to be seeing him too. But that small victory for Walter's sanity gets undone pretty quickly in "Subject 9," with a terrible one-two punch: Olivia's being stalked by an astral-projecting figure, which may be one of the Cortexiphan kids that Walter screwed up. And meanwhile, Dr. Sumner wants to re-commit Walter at St. Claire's, and it's up to Olivia to decide whether to send him back to the institution.

As Walter tells Olivia, right after schooling her on the proper way to eat a root beer float, he knows that he's in a precarious position. The Fringe Division employs him to help solve the weird-science stuff, but he's only kept around as long as he's needed. If his mental quirks and outright bouts of madness become more trouble than they're worth, the FBI can always ship him back to the hospital to rot. Hence Walter's decision to leave the lab and prove to Olivia that he can pull his own weight — in spite of all the unaccustomed stimuli driving him insane.

I sort of wish the show had spent more time, in its first few episodes, exploring this newly fragile version of Walter, and less time trying to make some probably non-existent newbie viewers comfortable with "freak of the week" episodes. But now that we've finally gotten the full-on portrait of a Walter on the verge of total mental collapse, it's a brilliant reminder of just how great John Noble is — not to mention Anna Torv, who does just as much heavy lifting in this episode, without being overtly showcased nearly as much.

While Walter is struggling to convince Olivia that his madness is manageable, and that everybody's a little crazy in their own way, he's also being confronted for the first time in years with the fall-out from the Cortexiphan trials. Even though Olivia remembers her experiences in Jacksonville, she doesn't seem to have met any of the other Cortexiphan kids as an adult, and the extent of the horror slowly unfolds all over again. The scene where Olivia and Astrid are looking through the Cortexiphan files and discussing the fact that Walter experimented on kids — and then Walter turns off the video link because he can't stand to hear that conversation — is quietly powerful.

And that's before we meet Cameron aka Mark, the guy who could astral project when he was a kid. As an adult, all he can do is cause metal to fly around scarily when he gets upset. (Like, when he's on a date that's not going that well, and he makes all the metal fillings fly out of his date's mouth.) Of all the Cortexiphan subjects we've seen before, Cameron might be the most unenviable, since he has no special powers to counterbalance his weird side effects. And Cameron delivers some of the most powerful rebukes any Cortexiphan subject has ever given to Walter, even as he begins to realize that Walter is no longer the terrifying figure he remembers from his youth. Cameron and Walter are both a matched set — two completely messed up people, linked by a shared history of child abuse. The abuser is no less ravaged than the abused.

Meanwhile, Olivia "was always the strongest" of the Cortexiphan kids, and they all looked up to her, as Walter says. And as Cameron puts it later, Olivia was Walter's "favorite."

The final bit of knife-twisting in the episode comes when Cameron begins to suggest to Olivia that her "astral projecting" stalker isn't anyone or anything else — it's Olivia herself, starting to have horrible side effects from her Cortexiphan treatments. Olivia herself brings up this idea at the start of the episode, and Walter ignores it but never quite dismisses it. While Olivia is debating whether to protect Walter from St. Claire's Hospital, Walter himself is trying to protect Olivia from the notion that she's starting to turn into a tormented basket case like Cameron.

So in the end, the blue energy field that's been stalking Olivia isn't Cameron, nor is it a side effect from Olivia's own powers. Instead, it's Dr. Manhattan. No, wait. Actually, it's Peter Bishop, finally back from limbo. And, as we'd sort of expected, Peter remembers everything about the "original" reality, but nobody else knows who he is or remembers all the fun times they shared together. Peter falls naked into the lake where his younger self drowned in this version of reality, and then he winds up in a hospital where the Fringe team assembles to confront what appears to be a major security risk. At last, Peter and Olivia are face to face — but she doesn't recognize her alternate-timeline sweetie.

So what are we to think of this version of Walter Bishop? Can we do anything but pity him, or see him as a manifestation of hubris gone wrong? There's just the merest sliver of playfulness and root-beer-float guru-hood to keep us from seeing the ugliest truth about Walter: that his outward stigmata reflect an inward, self-inflicted abscess at the core of his being.