Olivia's apartment. Olivia is reading Ella a bedtime story when she is interrupted by a phone call from Peter. He's actually calling for Rachel. Olivia is clearly a little uncomfortable by the two of them interacting socially, but hands the phone over and continues with the story.
Animal testing laboratory. FOUR ANIMAL RIGHTS ACTIVISTS break into a pharmaceutical testing facility. One of them peppers the walls with animal rights propaganda, while the others free animals from cages. A silent alarm is tripped.
Elsewhere, a man, presumably a SECURITY GUARD, is awakened by a text message informing him of the break-in.
One of the activists, JONATHAN, discovers a heavy steel door. Moments later, the security guard arrives and confronts Jonathan, praying that he didn't open the door -- but he did. Something behind the door grabs the guard violently and lifts him into the air. The activists, now in hysterics, flee for the exit. But Jonathan is snatched by the leg and dragged back inside. Helpless to save their comrade, the others rush to their vehicle.
As they speed away, they hear a familiar growl, and something hits their vehicle viciously from the side, and sends it tumbling end over end into a ditch. As the dust settles, the activists see something approaching the vehicle -- and their first glimpse of it is their last.
The Harvard Lab. Peter tries to poach part of Walter's omelet, only to discover a human ear growing inside of it. As an angry and disgusted Peter lectures his father on lab safety, Astrid interrupts and tells the Bishops they have a case.
At the crash site, Broyles briefs the team. Walter examines the dead and posits that whatever killed these people had enormous claws and fangs -- certainly nothing indigenous to the area. While Walter munches on fast food left in the wreckage, Olivia realizes there are four soft drink containers, but only three victims. She deduces that one of the bodies is missing.
Olivia arrives at the dorm of one of the identified victims, CHRIS HAWKINS. She asks his neighbor, CARL BUSSLER, to tell her about Chris. Carl reveals that Chris was involved with an animal rights group called Animals First.
Back at the lab, Walter and Astrid perform an autopsy on one of the victims. They discover large holes in the victim's back that Walter can only attribute to the bite of a very large snake. Even more perplexing, Walter extracts what appears to be a stinger from the victim. Olivia asks Astrid to find any animal testing labs near the crime scene that may have reported a break in.
The woods. An Animal Control van responds to a call reporting a "monster" in the forest. As the two officers enter the woods, they hear a rattle and a familiar growl.
At the lab, Olivia asks Peter about his phone call to Rachel. It was a casual conversation about a pop song; does that bother her? Olivia says no, though her body language suggests otherwise. Meanwhile, Walter shuffles through some of his old files. He pulls one out, and begins to read it looking visibly upset. Charlie calls to say he's going to investigate a monster sighting off Route 30.
Charlie arrives at the scene before Olivia. The Animal Control truck is now battered, the windshield spider-webbed and cracked. Inside, he finds the body of one of the officers and calls for backup -- then hears a guttural roar and a human scream from the forest. Charlie runs to investigate, but he's too late. As he nears the second officer's body, a tail drops into frame and slashes at him. Olivia arrives just in time to hear gunshots. She finds Charlie propped against a tree, wounded but alive.
Later, Charlie is examined by paramedics at the scene. Peter and Walter arrive just as one of the medics extracts a stinger from Charlie, which Walter collects. Walter questions Charlie about the beast's features. He believes they are dealing with a transgenic species; that is, a grotesque animal hybrid. Astrid calls Olivia with a lead on a testing lab near the original crime scene.
Swift Labs. The same lab we saw the activists break into, only now there are no signs of the attack. Olivia asks the head of the lab, DR. SWIFT, what sort of work is conducted here. He says the do cosmetic and pharmaceutical trials. Olivia tells him about the attack and grisly deaths of the animal rights activists, but Swift says he doesn't see how this is relevant to him or his lab. Olivia asks to look around, but Swift tells her that, for his clients' privacy, she needs to get a warrant.
At the Harvard lab, Walter confesses that twenty years ago, while working for Kelvin Genetics, he tried to create a creature like the one they're dealing with. But his efforts failed because of massive mutual rejection; the parts of his creations were incompatible. As a heated discussion begins between the Bishops, the body bag in the background begins to move as though something is creeping around inside it. Peter unzips the bag to find hundreds of maggot-like larvae crawling from the autopsy incision in the body. Then the body bursts open with thousands more larvae. Walter deduces that the stinger must carry the eggs and implant them in the host. As the implications of this news dawns on Olivia...
A tender moment between Charlie and his wife is cut short when the doorbell rings. It's Olivia, clearly here to deliver some bad news.
Back at the lab, Walter gives Charlie a sonogram and determines that he's been "impregnated" by the monster. Walter says they can't simply remove the larvae because they've already spread throughout his system. But Walter has another idea. He instructs Astrid to draw some of Charlie's blood; he's going to try to poison them.
Olivia determines that the larva burst from the first victim's body approximately 24 hours after being planted, so she estimates Charlie has about 16 hours left if Walter's plan doesn't work.
Playground. A mother chats on the phone while her little boy navigates his way around the jungle gym. Inside a giant plastic tube, we catch a glimpse of the monster's tail.
At the FBI, Olivia tells Broyles the monster hasn't been sighted in over 10 hours. Peter calls to say that the poison they used to dose the sample of Charlie's blood killed the larvae, but it also poisoned the blood. Walter thinks that the larvae act as parasites, extracting nutrients from their host. He believes if they can secure some of the monster's blood, they could transfuse Charlie with it. The larvae would recognize it as their mother's, stop feeding, and starve themselves to death. This means they need to catch or kill the beast.
Back at the playground, the child comes face to face with the monster in the tube just as his mother pulls him out.
At the Harvard lab, Olivia and Peter deduce that the monster is traveling underground and surfacing to feed and mate. Charlie, who's now laid out on a gurney, screams in agony. His stomach undulates; the larvae are growing.
Later, Olivia receives a call from Carl Bussler. He says another classmate who was known for being an animal rights activist hasn't shown up for class in a few days. His name is Jonathan Swift.
Swift Labs. Olivia barges in unannounced on Dr. Swift and reveals that she knows his son was killed by his creation. Swift tries to deny it, but as the gravity of the situation lands on him, he comes clean and takes responsibility for creating the monster.
Olivia updates Astrid over the phone as Swift is arrested. Swift was collaborating with a geneticist named Cameron Deglmann, who turns out to be the security guard who was killed the night of the break-in.
In the Harvard lab, Astrid relays some good news. Olivia was able to get a list of all the animals that were used to make the monster -- one of them being a species of bat whose unique immune system allows it carry diseases without being affected by them. Walter surmises that this same immune system is the key ingredient that allowed the hybrids to survive without rejecting their component parts.
Walter also says that bats are able to track their offspring from miles away, so he suggests they go to the sewer and lure the monster with its own larvae and kill it. This means, of course, that they'll need to put themselves in harm's way to draw it near. With no other options and time running out, the group consents.
Olivia assures Charlie that they'll be back shortly with the proper ingredients to poison the larvae in his body. Then Olivia, Peter, and Walter go down into the sewer to bait the monster with a container of larvae. On the way out, Walter pockets a container of the poison he used earlier.
Meanwhile, Charlie calls his wife to hear her voice, possibly for the last time.
In the sewer, Walter excuses himself to use the "facilities." Once he's away from Peter and Olivia, he locks himself behind a large metal storm hatch to separate himself from them. Peter tries to break through the gate, insisting Walter open it. Walter says it's time that he took responsibility for his actions. Knowing that the beast might eat him, Walter swallows a vial of the poison to ensure that he kills it. He says that in the event that he survives, he'll need to be dosed with an antidote back at the lab. Then he turns and continues alone down the tunnel.
Walter reaches a dead end and freezes up when the monster surprises him. Olivia and Peter, who have managed to break through the gate, round the corner just in time to distract the beast. It lashes at Walter with its tail, and then lunges at Peter and Olivia. Gunshots ring out -- the monster hits the ground and rolls to a stop in front of a frozen Peter and Olivia. Then they see Walter leaning against the wall, a smoking gun in his hand.
Back at the lab, Astrid injects Walter with an antidote for the poison he swallowed. Walter connects an I.V. drip of the monster's blood to Charlie. Peter tells Walter that he was brave today. Later, Walter gives Charlie another sonogram, and much to everyone's delight, the larvae have begun to die. The plan worked.
It's late when Charlie gets home. His wife's already asleep as he slips into bed next to her.
Rachel and Ella are passed out on the couch when Olivia arrives home. Olivia goes to bed with the lights on.