Bolivia picks up Frank, her virologist boyfriend, at the zeppelin airport. They reunite cutely. In the airport bar, an older man sneakily swaps glasses with a younger guy, who's soon vomiting in a restroom. Something's moving under his skin. The older man enters the stall and emerges with a bottle of bugs. Others scurry on the floor.
Frank notes that Bolivia was quiet on the drive back. And she seemed a bit distant, almost not herself, when he'd call from Texas. While stealthily closing Peter's file, she says that things have just been a bit overwhelming since Broyles disappeared. Lincoln has taken over, but they've stopped the search. Frank sympathizes, and they make out. Later, he asks her to go away this weekend. She'd love that.
Fringe Division investigates the airport dead guy. These mysterious bugs ate their way out from inside him. Eventually, Charlie and Bolivia learn from insect scientist Mona Foster that they're skelter beetles, parasites that lived exclusively in sheep - which died out ten years ago.
Brandon shows Walternate a breakthrough in the experiments with the chemical synthesized from the other Olivia's brain. One subject began levitating objects, before having a seizure. Walternate wonders why, after nine subjects died 30 minutes after injection, this one became telekinetic. Brandon thinks it's because he was the youngest, and they should experiment on children. Walternate says that's NOT an option.
In a cramped lab, the man from the airport experiments on a beetle, trying to detect some kind of enzyme. He's highly frustrated by the negative results.
Frank's at Fringe, helping to determine if this beetle thing is an epidemic, until Bolivia and Charlie bring the information about the skelter beetle. Frank wonders why the beetles, which had a symbiotic relationship with sheep, are now attacking their host. Farnsworth suggests using a Fringe Alert to see if the public knows anything.
Frank privately asks Lincoln if Bolivia can have time off. Lincoln asks why, and Frank says can you keep a secret? After Frank confides he's going to propose to her, Lincoln immediately blabs it to Bolivia.
Charlie fields a call from a scientist, who knew a skelter researcher named Dr. Armand Silva. He was seriously attached to his work. After the bugs died out, he tried to kill himself.
But now Silva - who has major delusions of grandeur, comparing himself to the likes of polio-vaccine inventor Jonas Salk - is continuing his work. He doses a guy at a diner, and soon he's collecting more beetles from the corpse. The bugs are bigger now.
In a cozy bedroom scene, Walternate confesses self-doubt to a woman named Reiko. Even with their world's existence threatened, there are lines he cannot cross. Is he weak? He's afraid he's failed. Peter was here of his own choosing, as he needs to be, and Walternate lost him. He underestimated Peter's attachments in the other universe, and hadn't factored in the girl. Reiko knows Walternate's strong and will find an answer. They kiss.
Frank tells Bolivia that, in '97, Silva was supposedly close to a vaccine for avian flu, based on an enzyme the skelter beetle produced. Although Silva could bring back the extinct beetle by cloning a preserved specimen, its eggs wouldn't hatch without sheep. If he's re-engineered them, that could explain why human hosts die. Based on the size difference between the two bug batches, he's probably been modifying his work.
Bolivia shares this with Lincoln, then heads out. Frank decides to pop the question right then. She says yes. Lincoln calls: They've linked Silva to a Brooklyn warehouse. Meeting him there, she tells him about the engagement. They hear a sound and split up. Lincoln investigates a walk-in freezer, and someone locks him in! He can't get a signal on his communicator. Meanwhile, Bolivia is startled, stumbles backward - and crashes through the floor, into a lab below.
When she comes to, Silva is making her drink water. She's handcuffed to a chair. He rambles insanely, lamenting that the world changed and robbed him of his legacy.
Lincoln breaks the freezer's lock with liquid nitrogen, then calls for backup. In his lab, Silva explains that he needed one more human host to incubate a queen beetle, because she can hatch her own eggs. Then he'll get what he needs for his vaccine. Alarmed, Bolivia yells, "What did you give me?" She retches. Lincoln bursts in. Bolivia says don't shoot him; they need his help. She's infected.
Charlie, Frank, and the tactical team rush in. In the ambulance, Frank prepares to give Bolivia a powerful anti-parasitic that will kill the bugs but ravage her system. The EMT sees movement on the ultrasound. Meanwhile, Silva confesses Bolivia's not the host - he is. The EMT stops Frank from injecting Bolivia. She isn't infected. She's pregnant.
Silva pulls the queen from his neck. As he dies, he tells Lincoln to make sure they spell his name right.
Frank visits Bolivia in the hospital. She says the baby's healthy; her fall and the adrenaline triggered some kind of morning sickness. How far along is she? Six weeks. He does the math, asks if she's in love with him, the father? She doesn't answer. He turns away. "Frank," she says. "You were gonna marry me," he replies. And leaves.
Walternate visits Bolivia. He knows about her pregnancy, and she'll have whatever resources she needs. After all, she's the mother of his future grandchild. He smiles.