Olivia's at a restaurant with Nina, who says that Massive Dynamic is completely reevaluating their security measures, after what that woman did (meaning the other Nina). Olivia says she's in love with Peter. It sounds absurd, but it's like she's known him her entire life. She doesn't understand what's happening, and she's scared.
When Nina reassures her, Olivia gratefully says that they should do this more often. Surprised, Nina says we always have breakfast on Saturday mornings. Doesn't she remember? Now Nina's worried. Olivia will talk to Walter.
A woman named Jane Hall returns home after her husband Mark's funeral. A badly scarred man named Anson Carr dabs a substance on his neck and grabs her. At first she's terrified, but suddenly she relaxes. They kiss. Then he suffocates her and takes a sample from her skin.
Olivia visits Walter, who shows her the teddy bear camera he bought to monitor the cleaning crew. It inadvertently recorded the event with September and Peter. He thinks a tiny blip is something undetectable to the human eye. Astrid brings Walter a device for drastically slowing down video. Olivia's phone rings.
The team meets Broyles at Jane's place. She has marks on her skin - the second killing in a month where a woman had such burns. Both were recently widowed, and their husbands died mysteriously: Their bodies were completely dehydrated. In both cases, the deceased husbands' DNA was found on the victims' necks.
Walter phones Peter and stops him from getting on a bus to New York - to stay away from Olivia, like Walter wanted. He plays Peter the slowed-down video showing the other Observers retrieving September, and a flash of light flowing from September to Peter's eye.
While preparing to examine Peter's eye, Walter says leaving Olivia was ethically right, but he doesn't know if he could've done that. Peter's a better man than he is. Startled, Peter speaks the Greek phrase his mother always said to him in the other timeline, that means "Be a better man than your father." He finds it strange to hear Walter say that.
Walter removes a tiny black disk from Peter's pupil, with an address on it: 228 1/2 Morrow Street. Walter says that this message would have dissolved and rooted itself in Peter's mind's eye, compelling him to go there.
Carr cleans up a bloody MRI tube in his lab and distills some substance, sniffing it ecstatically.
Walter autopsies the Halls and explains that, among other things, pheromones are lost when a body is as severely dehydrated as Mark's. The compound on Jane was a concentrated solution of Mark's pheromones, which caused the burns. Pheromones control our moods, our appetite, who we give our hearts to. Lincoln sneaks a glance at Olivia. And by all reports, the victims had great marriages.
Peter goes to the address and discovers that it's the Observer's apartment.
Privately, Olivia tells Walter her memories are slipping. She didn't care before, because Peter was going to be with her. But it's getting worse, and she wants to go back to who she was.
Carr trolls for another couple in the park.
Walter explains how, in perfume making, one disparate rancid note is used to counterbalance all the agreeable scents in a fragrance. Similarly, their perp's compound contains a rancid note: castory, an extract from beaver glands. Only a handful of perfume manufacturers use it - and, it turns out, one recently had an employee arrested for stealing the stuff. That would be Carr, who's cooking his latest victim, Andrew, in the MRI tube. But he's gone when the Fringe troops burst in. They rush to save Andrew's wife, Diana.
Peter finds a briefcase containing the Observer's devices, including a beeping compass. He takes the briefcase and follows the signal to the woods. Suddenly the beacon, that mysterious cylinder first seen in Season 1's "The Arrival," drills up through the ground.
While waiting to apprehend Carr, Olivia talks with Diana about love . . . and realizes Andrew was having an affair. The team storms into the other woman's house and nabs Carr before he kills her.
Carr tells Olivia that if he'd succeeded, he could've given the world what she has: Love. He can smell that she's in love.
Olivia visits Nina and says today she met a woman who had let go of the possibility of finding love. Olivia saw herself in her - and didn't like it. All these memories are from a better version of herself, so she's decided to let things run their course. Nina's stunned. Olivia says when the day comes, if she doesn't remember this, Nina should try to build something with her again. They say they love each other.
Peter's examining the cylinder at his house when light bursts from the top. He runs upstairs and sees September, who says thanks for helping me come back. The other Observers hid the universe from him; they locked him out. He needed Peter to activate the beacon so he could return. Peter begs September to help him get home, and the Observer says you have been home all along. There's no scientific explanation, but September thinks Peter couldn't be fully erased because the people who care about him wouldn't let him go, and he wouldn't let them go. September believes they call it love. Peter asks, and Olivia? She is your Olivia, says September.
The house shakes. September disappears. Peter runs downstairs. The cylinder is gone, and there's a hole in the floor.
Arriving home, Olivia sees Peter on the street. They rush toward each other and kiss.