Last night's Fringe episode may have pulled off something that's never been done before: a lengthy flashback about events that nobody remembers. We weren't delving into the memories of any particular character, because they've all blotted it from their minds.

Which makes it even more fascinating that it felt like we were seeing defining moments for all three of our main characters. They were making choices (and mistakes) that continue to haunt them — but as far as I can tell, none of these three characters remembers anything about the events of this episode. It makes you realize just how much Fringe is a show about repression, both repressed memories and repressed trauma — and the ways in which repressed memory is the constant companion to Walter Bishop's fevered search for knowledge. The more you know, the more horror you have to forget.

Spoilers ahead...

At least, I don't think Walter remembers anything about the events of "Subject 13." Unless I'm mistaken, he's been pretty clear in the past that he doesn't remember that Olivia was one of his test subjects in Jacksonville. Or at the very least, he's a bit hazy on what happened back then — during the "Jacksonville" episode last year, Walter seems to have some grasp on the fact that Olivia was the most promising of his Cortexiphan subjects back in the day. But at other times, he's seemed as surprised as anybody to find out what he did to her. No doubt those missing pieces of Walter's brain contain some of the memories from this era — but feel free to correct me in comments as to just how much Walter seems to remember of the Jacksonville days.

Still, even if Walter might have some memory of the events of this episode, the same can't be said for Olivia and Peter, who've both repressed it totally. As far as they know, they never met before a few years ago, and Olivia is still hazy on what Walter put her through together a quarter century ago. The fact that both Peter and Olivia have managed to forget such significant events in their lives is either a convenient plot device or a commentary on the ability of children to bury unpleasant or destabilizing memories — and I choose to view it as the latter.

The thing I really loved about this episode was the fact that we gained more insight into how Walter went from the well-intentioned cross-universe kidnapper we saw in "Peter" to the ruthless experimenter on children we've glimpsed elsewhere. But this episode didn't take the easy route and give us a rationalization that would let Walter off the hook — we understand Past Walter better, but he's still kind of a monster.

It all stems from the fact that Young Peter realizes he's in the wrong universe, even if he thinks he's passed through some kind of magical gateway at the bottom of a lake. And in a scene that I'm still a bit surprised they were allowed to show on network television, Peter ties a cinderblock around himself and tries to drown himself in that frozen lake, trying to get home. (I know you can show all sorts of things nowadays, but kids attempting suicide seems right on the edge, especially for a show that's trying to attract more younger viewers.)

So no matter what the Bishops do, they can't convince Peter that they're his real parents, and they're scared to tell him the truth. He keeps clinging to the truth that something about this world is wrong, and it's not just because he's been sick. Elizabeth Bishop manages to get a smile out of him by taking him toy-shopping, but all of her talk about how that field of white tulips prove that you can change the world with your imagination leaves him mostly unimpressed.

It's interesting that Peter was so good at telling the duplicates from the real things back then, but Fauxlivia fooled him completely. I guess parents are different.

The defining moment for Peter in this episode comes towards the end, when he's found the runaway Olivia in that field of white tulips. And he's trying to comfort Olivia, so he reaches for the thing his fake-mom said. And he starts to call her his mom, then checks himself. Then he goes ahead and says it anyway — because if that's not his real mom, then he can't help Olivia. He can't offer her comfort or help unless he's got a place to stand, an identity of his own, and that means he has to make the compromise and accept the fake-mom as his mom. And if he keeps rejecting Walter as his dad, he can't vouch for Walter's willingness to hear about Olivia's problem. So Peter's compassion forces him to sell out and accept what he knows, deep down, is a lie. And once he's made the compromise, he accepts it so deeply that he doesn't even remember it later.

Because Peter is having such a hard time adjusting, the Bishops are desperate to send him home — but Walter fears for the stability of the multiverse if he builds another world-traversing machine like the one he originally used. Hence the Cortexiphan trials, which have two purposes — figuring out a way to send Peter back, and preparing for the retailiation if Walternate realizes where his son went.

And when Walter realizes that Olivia crossed over to the universe when her stepdad hit her, he sees a chance to fix everything. He's so eager to undo his earlier mistake, his first impulse on encountering an abused little girl is to terrorize her further.

This might be the most monstrous entry in Walter's long catalog of monstrosities: his willingness to take an already damaged little girl and see how far he has to push her, before he chases her out of this world. The sequence where he tries joy, exhileration, frustration, loneliness, and finally fear, is a weirdly hilarious spin on television's traditional "running some tests" montage, that gets creepier and more horrible as it goes along. Until finally, Olivia does the pyrokinetic thing. And Walter concludes that what he really needs is a mixture of love and raw terror.

Even after Elizabeth Bishop points out how unethically Walter's behaving, he seems unmoved — at first, he shrugs off her concerns by saying that she's reading lab notes that she shouldn't be looking at. And then, he does take her concerns on board, but says that to save the countless people who could suffer from Walternate's recrimination, torturing Olivia "would have to be considered."

Finally, Walter does do the right thing, telling Olivia's stepdad to back the fuck off or Walter will put him in a hole — but it's too little, too late. And we already know from past episodes that this is just the beginning of Walter's long string of psychotic experiments on Olivia. And tons of other children.

Olivia's defining choice, meanwhile, is to decide to trust "Dr. Walter" with her secret, that her stepdad really is hitting her and that she doesn't want to go home to him. The sad part is, Walter really seems to have formed a bond with Young Olivia, and is the only authority figure she trusts. Of course, it's not entirely Walter's fault that when Olivia finally shares her pain with Walter, she picks the wrong Walter.

One of the other neat things about this episode is that it plugs a plot hole that nobody, to my knowledge, had ever brought up: Exactly how did Walternate figure out where his missing son went? Even if he's a mega-genius, he wouldn't necessarily leap to the idea that his alternate-universe self appeared out of nowhere and stole his son away. And indeed, we see Walternate groping for an explanation through a haze of booze fumes — was it plastic surgery? A shape-shifting alien? Something else?

The irony is that if Walter wasn't so certain that Walternate would know what had happened and retaliate, Walternate might never have worked it out. Walternate only learns where his son has gone when Olivia, thanks to Walter's experiments, crosses over and appears in front of Walternate, conveniently blurting out enough information about other universes to let Walternate put the pieces together. She even leaves Walternate her sketchbook, with a picture of Peter and herself together.

So what did this episode add up to, besides filling in some backstory? I think it was important for a couple reasons. First of all, the decisions that people made in this episode are still resonating today, and in many ways these characters are repeating their past choices. Walter is still too quick to choose the path of expediency, for all his claims that he's changed. But also, he's still eager to sweep unpleasant truths under the rug, the way he did the truth that Young Peter was in the wrong world. As for Olivia, she's still making the choice to trust Walter, and to rely on him as an authority and a source of knowledge. And Peter is still making compromises in the name of being able to protect others.

And second of all, it underscores that Walter really did start this war between universes. He not only instigated it by stealing Peter, but he also started preparing his child army before Walternate even had an inkling what had happened.

And it makes you wonder just what other misjudgments from these people's pasts remain to be uncovered, like unexploded munitions under a busy street.