What makes you, you? Is it your DNA, your brain chemistry, your upbringing, your memories? If your life had been changed in an innumerable amount of tiny ways, would you still be the same person?

These are the questions that season 4, episode 2 of Fringe, “One Night In October,” attempts to answer.

It probably goes without saying, but the following contains major Fringe spoilers!

As the episode opens, we’re treated to a dank, dark room, where two men are talking. One is strapped into a chair, terrified. The other is asking him about his happiest childhood memories. It’s slowly revealed that both have tubes of blue liquid surgically inserted into the back of their skulls, leading to a home-made device.  As the camera pans away, the victim’s face, including a single tear, freezes solid. Cue the nifty new orange title sequence.

Back in the lab, in the primary universe, Walter continues to struggle to focus on his work. While Astrid attempts to calm his ranting at the members of the alternate universe, Astrid and Olivia have a frank discussion about her love life, or lack thereof – and, apparently, Walter has as hard a time remembering Agent Kennedy Linclon’s name as Astrid’s.

In the FBI field office, Broyles describes to Olivia dozens of victims killed in the process seen in the pre-roll. Their brains were frozen solid with electrical discharge damaging the tissue around the traumatized area – and the holes in their skulls didn’t help either. When Olivia asks why she hasn’t heard about this yet, Broyles tells her that the victims in question are from the other side, and this will be the first case handled by both Fringe divisions in a joint operation.

Broyles and Olivia travel to the bridge facility where they meet with the alternate Olivia (Bolivia) for a debriefing. She describes a brilliant serial killer, John McClellan (guest star John Pyper-Ferguson, Drive) who’s been on the run from the FBI for five years. Bolivia wants to take the same man from the primary universe into the alternate one, to see if they can use his insight (into his alternate self) to track down the killer. The lack of trust that Olivia has for her alternate, and by extension the rest of the alternate world, is on full display.

Olivia goes to visit the primary-universe McClennan. In our universe he’s a mild-mannered  forensic pathology professor who, appropriately, is a world-renowned expert on serial killers. To keep the secret of the alternate universe safe, they politely drug McClennan to take him through the bridge.

When they reach the alternate universe, Professor McClennan is handed over to Bolivia, who’s dressed up to look like her primary counterpart. (She should be good at it by now.) The killer McClellan is seen stalking a mother and her young daughter at a gas station.

Back in the Harvard lab, Walter recreates the famous Maxell hi-fi ad with a bit of Mozart. It’s clear that he’s falling apart mentally, even more so than usual. Professor McClennan and Bolivia investigate killer McClennan’s home, where he describes his own alternate as a brilliant but deeply flawed individual. The killer had a horrible childhood. They discover that the psychopath has a fixation on the structure of the brain, and he’s ordered parts from various foreign countries for a device of unknown function.

At the gas station, killer McClennan deftly kidnaps the anonymous woman and steals away with her to an unknown location. Bolivia and professor McClennan continue to explore the house, where they find a wall full of photos of smiling people. The professor concludes that the killer kidnaps people at their happiest moment. When the professor discovers a chair that used to be in “his” old house and a picture of his own father, he panics, running outside to see the Olivia… and a neighborhood that’s been buried in the temporal bonding agent, Amber.

Olivia explains the truth to professor McClennan, who is understandably upset that his alternate self is a serial killer. McClennan extemporizes on how he and his alternate self are different – and alike. He himself has dark, violent urges stemming from an abusive childhood. But he was taught to control them through the kind influence of a woman called Marjorie. The professor explains that he studied the brain to try and help people like himself. McClennan concludes that his alternate self had no Marjorie to help him, and instead of feeling angry at his murderous actions, he feels pity and a desire to assist (himself).

The Fringe division then learns that killer McClennan has captured another victim (the woman at the gas station). When they go back to retrieve the professor, he’s escaped, presumably to find his alternate self before the Fringe team does.

Alternate Lincoln and the Olivias rush to the field office, where Alternate Broyles is, without explanation, alive, well and whole (seriously, shouldn’t he be extremely dead after being zapped between universes? More fallout from Peter’s actions). Alternate Astrid uses her super-Aspergers powers to find out where, in all probability, the professor is. Olivia uses the photograph of McClennan’s father to track down a secondary (and correct) location at an abandoned farm.

The professor finds and confronts his alternate self just as he’s about to drill into his new victim’s skull. He tries to convince his panicked alternate that there’s another way, a method of suppressing his dark instincts with self-control. He describes the night that his abusive father found his stash of slaughtered animals when he was a boy. While Professor McClennan ran and found his salvation in Marjorie, the killer didn’t, and continued his life of abuse. The professor tries to convince the killer that he can help, but instead of accepting, the killer knocks him out and hooks him up to the machine.

The killer activates his machine. As the professor slowly recounts his happy times with Marjorie, the killer experiences the same memories, physically drawing them out of his alternate’s brain and into himself. In a few moments he experiences years of life-changing kindness and understanding.

After some wrong turns, the Fringe team finds the killer McClennan dazed, processing the memories he’s stolen from himself. He feels instant remorse for the first time in life, and concludes that these memories are the only thing that separates the two of them – and now, he suddenly feels an intense empathy and sorrow for his many, many victims. The killer can’t handle the grief and shoots himself while Olivia watches.

The primary Fringe team takes Professor McClennan back to their universe to recover. The damage to his brain is not fatal, but he can’t remember the last few weeks – or his experiences with Marjorie. Olivia and Broyles are worried that without those positive memories, he’ll become just like his alternate self and begin killing people.

While McClennan can’t remember Marjorie, he feels her influence on her life. In the same way that the addition of Marjorie to the killer’s psyche can’t overcome his lifetime of taking and giving pain, the subtraction of Marjorie from the professor can’t undo the good that she put into his life. Broyles observes that there are people who leave an indelible mark on a person’s experience, whose influence cannot be erased.

…and for Walter, that person is Peter. As he sleeps in his lonely apartment in the Harvard lab, he continues to hear Peter’s disembodied voice. A voice that he can’t identify, that he doesn’t remember… but that he can’t shut out, either. The stress drives his fragile mind further into darkness.

The episode is a return to form for Fringe, and frankly, a much mor engaging forty minutes than the season premiere. The observations on human nature, that people are not just the sum of their parts or even their experiences, are definitely some of the more subtle and heartfelt moments that the show has had for some time – and while there isn’t much to continue the universe-hopping arc (or explain Peter’s disappearance and the full scope of what it’s done), there’s just enough to keep me excited for next week.

Fringe airs Friday nights at 9 PM on Fox.

Follow Michael on Twitter: @MichaelCrider