Olivia is reading stories to Ella for bedtime when Rachel tries to get her daughter in bed. Peter calls Olivia, who worries that something is wrong. Peter assures her there isn't and he called for Rachel. Rachel takes the phone, asks if he remembered "it," and laughs. Olivia wonders what's going on as Rachel leaves the room, but continues reading to Ella.
Animal activists break into a laboratory to free the animals and one of them, Jonathan, opens a secure door. They set off an alarm and a man at home, Cameron Deglmann, gets the signal. As he drives to the laboratory, Jonathan enters the vault. The man goes inside and orders them to stop, then realizes Jonathan has opened the vault door. Jonathan comes out and Deglmann tells them all to get out… and is then yanked inside the vault. The activists run for it but Jonathan is grabbed and yanked inside. The others drive away but the creature from the lab catches up to them and they go off the road. All three of them survive the crash, but the creature attacks them.
The next morning, Peter tries to take a bit of Walter's omelet, but his father explains there's an ear inside. Walter was using it as an incubator and the ear was growing. Father and son argue but are interrupted when Astrid tells them that Olivia needs them in the field. They go to the car crash and Walter determines the victims were killed by some giant creature with claws similar to an eagle or a pterodactyl. Olivia determines that there were four people in the truck but only three corpses, and checks the local restaurant where they ate near MIT. She tracks one of the activists, Chris, via the student database but his roommate Carl doesn't know where Chris is. He does say that Chris was an animal activist.
At the lab, Walter determines that the victims were apparently bit by a large snake. As Olivia returns, Walter suggests that the wounds indicate several animals attacked the activists. Olivia and Astrid start checking animal labs in the area to see if there were any break-ins. Walter removes a spine from the corpse, and seems to recognize it.
An animal control truck gets a report of a "monster sighting" and the two men drive into the wood to recover it. They spot something in the woods.
Olivia asks Peter what he was talking to Rachel about, and he admits that they're friends. Olivia says it doesn't bother her. Charlie calls with news about the animal control report. Meanwhile, Walter goes through his old files and finds something disturbing. Olivia leaves to help Charlie and Walter warns her to be careful. Peter wonders what's going on but Walter says he just wants her to be safe.
Charlie finds the animal control truck and one of the men, dead. He calls for reinforcements and hears something moving in the woods. He goes to investigate and finds the second dead man… and something attacks him. Olivia hears the gunshots and runs into the woods and finds an injured Charlie. As the EMT removes stingers from him, Walter questions Charlie and asks detailed questions about the creature. Olivia asks him what he's thinking and Walter explains it's a transgenic species, an animal created from the genes of multiple species. Astrid calls to tell her they've located a lab near the crash scene.
Olivia goes to the lab and talks to Dr. Robert Swift, the head scientists, who says they conduct animal trials. She explains that the activists died, possibly after breaking into the lab and freeing the animals. Swift assures her there was no break-in but Olivia notices the staff is doing clean-up work. She asks to take a look but Swift says that she needs a search warrant.
Olivia returns to the lab to ask Walter what Swift would need to do the necessary genetic work. It's clear that Walter is more distracted than usual, and Peter agrees to do what he can. Walter shows him a book containing drawings of a creature he tried to make 20 years ago. His creatures didn't survive, but he was doing fied work with Kelvin Genetics for the army. They're interrupted when one of the body bags starts moving. They open the bag and find it crawling with larvae. The corpse splits open as the larvae grow, and Walter realizes the stinger transferred eggs. Olivia remembers that Charlie was stung.
Charlie is at home with his wife when Olivia arrives to give him the news and take him to the lab. Walter runs an ultrasound and finds the larva inside of Charlie's test. It's impossible for him to remove them normally. Walter tries to come up with an idea and see the larvae from the corpse. He has Astrid take a sample of Charlie's blood while he prepares to inject a poison, trichlorimide. Olivia figures they have 16 hours left until the larvae come to term.
At a park, a boy Tuck gets on a play structure, unaware that the creature is hiding in one of the tubes.
Broyles hands over the files on Kelvin Genetics to Olivia, but there's no connection to Dr. Swift. Peter calls to tell Olivia that the trichlorimide started to kill the larvae but also is killing Charlie because they're parasitically bonded with him. Walter thinks they can inject Charlie with blood from the creature and convince the larvae not to feed on Charlie.
At the park, Tuck's mother notices something odd and pulls him out of a tube just in time.
Olivia and Peter go over the reports and wonder why something so large hasn't been seen more often. Walter gives Charlie a sedative and apologizes, then starts to explain he's responsible. Before he can finish, Olivia realizes the creature is traveling through the sewers. Walter isn't relieved and walks out. Before Peter can go after him, Charlie screams in pain as the larvae grow even further.
Olivia gets a report on the park sighting, but it doesn't correspond to the other patterns. Carl calls to tell her that Jonathan has disappeared, and his last name is "Swift." Olivia goes to see Dr. Swift, who insists his son wasn't involved. Olivia accuses him of creating the creature and says his son and Charlie are both in danger. Swift admits the creature killed seven people. Olivia calls in the police and then calls Astrid and explains that Swift has confessed he was working with Cameron Deglmann, who has no connection to Kelvin Genetics and died at the lab. Olivia passes on a list of the animals that were used, and one of them is a bat. Walter and Peter realize that the bat gene is the missing ingredient that allows it to survive. However, they still need a specimen of the creature's blood to save Charlie. Astrid suggests the use themselves as bait, but Walter has a better idea. The bats are maternal and they can use the larvae as bait. Walter thinks they can kill it with incendiary rounds.
As the team prepares to leave, Charlie says it's not a good idea and tells her not to get hurt o his account. She points out he'd do the same for him. Meanwhile, Walter hides a vial of the trichlorimide before leaving.
In the sewers, Walter places the larvae at a central point sot he creature can hear them. Meanwhile, Charlie calls his wife and laughs at her bad joke, and assures her he'll see her soon.
As they wait for the creature, Walter starts singing the song he used to sing for the family dog. Peter asks him to stop and Walter admits he has to pee. He goes off on his own and then closes and bars the gate behind him. He tells Peter that he has to deal with the creature on his own without anyone else getting hurt. When Olivia points out he's outmatched, Walter drinks the trichlorimide and says they have an hour to get him the antidote from the lab. If the creature eats him, it'll die.
As Peter tries to break through the gate, Walter makes his way through the sewers. The creature lowers itself from the ceiling just as Peter and Olivia arrive. It charges peter and Walter brings it down. They get Walter back to the lab and administer the antidote, and he then uses the creature's blood to make the antidote for Charlie. As they wait to see if it works, Walter admits to Peter that he doesn't think of consequences. Peter says that he was brave today. They perform a new ultrasound and confirm that the larvae are dying.
Later, Charlie goes home and kisses his wife.
Olivia comes home to find Rachel and Ella asleep on the couch. She turns off the light and goes to bed… then turns the light back on.