Animal rights activists break into a testing lab to free all the innocent beasts therein, only they open a door to an animal's cage, one who's really sick of having its eyes jabbed with mascara brushes. It violently attacks and kills the lab worker who responds to a tripped alarm, kills one of the activists on site, and then chases down and kills the other three activists, even though they're in a car.
So the Fringe team investigates, because the wounds indicate some kind of freaky animal not indigenous to the area (fang marks four full inches apart, huge claws), and they hit on the idea of some sort of genetic hybrid. Naturally, Walter did some work in that vein a few decades ago, so he thinks this is one of those "someone carried on my work" cases.
So he feels guilty when Charlie is hurt checking out an Animal Control report of a "monster" sighting. Worse news: one of the dead activists busts out all over with the larvae of the unleashed creature. Seems the monster lays its eggs in its victims using its tail. An ultrasound confirms there are plenty of the little critters swimming around Charlie's insides, and the best idea Walter has for getting rid of them is to inject Charlie with the blood of the creature, because his theory is that if the parasitic larvae think they're feeding off one of their own, they'll stop feeding, and die.
So no problem: just catch this thing, right? Like in The Host. Olivia's investigation discovers one of the activists was the son of a geneticist at a nearby lab. He tries covering up the genetic experimentation, but soon withers under Olivia's sunny determination, and provides the team with the monster's genetic makeup. It's part bat, which are highly maternal creatures, so Walter and the team head into the sewers -- which is how the animal's moving throughout the city -- to bait it using its own larvae.
Walter, despite finding out his work was not to blame for the current monstrosity, recklessly tries to take care of the creature himself, by ingesting poison that will kill the creature if he's devoured. Fortunately, the animal winds up being brought down by a couple of gunshots, and the blood is used to cure Charlie, who can go home to his wife and her terrible jokes.
Oh, and Peter and Rachel are spending time chatting on the phone, about -- among other things -- that ridiculously annoying "if you like pina coladas" song, which might be the weirdest aspect of the whole episode.
So Olivia's reading a Burlap Bear book to Ella -- like, nice to see Rachel's apartment hunting is going so well -- and Rachel comes in to say it's time for bed, and Ella whines and says this is the last book, which her mom reminds her is what she said about the last book, and then there's Olivia saying, "We're almost done," like, way to undermine your sister. "You two work well together. And I don't like it," says Rachel.
The phone rings, and Olivia's got the cordless in bed with her and Ella, and it's Peter, so Olivia assumes something's wrong, and he says there's not, and then he asks to speak to Rachel, which throws Olivia for a loop. She conceals her surprise, and Peter bashfully says it's something stupid and he can call back tomorrow if they're in the middle of something, but Olivia passes the phone over to her sister, who's all, "Yeah, did you remember it or not?" And then she's laughing, all, "Are you serious?" and walking away with the phone, and Olivia watches her go, like maybe Peter's calling Rachel because she sounds happy to hear from him instead of going all angel-of-death "what's wrong?" on him. And Ella has to ask her Aunt Liv to keep reading the damn book already, and Olivia continues on with the story of the Burlap Bear not believing there's a monster just down the road. "Aunt Liv? Monsters aren't real, right?" And Olivia, instead of saying, "Man, the shit I could tell you..." is all, "Of course not."
Quick cut to a bunch of animals in cages, monkeys and birds and rats, in a dark lab going completely bonkers. Four animal-rights ninjas wearing black and flashlights on their foreheads bust in and spray-paint the security cameras and open the animals' cages. Which I'm sure they appreciate. I mean, shouldn't these people be taking these animals with them? Instead of, "Here you go, chicken. Now you can run around the floor of the lab instead of staying in a cage in the lab"?
One of the activists is calmly spray-painting something on the wall, and refusing to be rushed by one of the other activists, who's worried that there's a backup system. "No, there's not," says the first guy. He tells the other one to finish up in there, and says he's going to go check out a door that has red light shining through the door's window. He smashes the padlock keeping the door closed and walks in, oblivious to a blinking red light and an alarm now going off. And I have to say that this seems kind of foolhardy to me. I mean, it's a lab with caged animals. The bigger the cage, the bigger the animal, right?