FROM ENTERTAINMENT REPORTER KIMBERLY ROOTS: A group of college students trash Swift Research, an animal testing facility, and then all but one makes a run for it. A girl implores the straggler, Jonathan, to hurry up, but he heads for a big metal vault with a red light flashing over it. Opening the door triggers a silent alarm that registers on the Blackberry of a man sleeping in his home; he panics and jumps in his car, speeding to the research facility. Shortly after reaching Swift, the man—and Jonathan—are killed in hideous fashion by something large, powerful, and unseen in the vault. The other protesters get out as quickly as they can, but their car comes under attack by what seems to be the same thing that killed Jonathan and the man. We don’t see it, but from the look of horror on the girl’s face the moment before she dies, it seems pretty horrible.
Phillip Broyles, Peter and Walter Bishop, Charlie, and Olivia survey the crash scene the next morning. Charlie notices that there’s fast food in the car: four drinks, but only three clawed-up bodies—someone’s missing. Peter IDs the food as coming from a dive near the Massachusetts Institute of Technology. Because none of the victims are carrying identification, Olivia says she’ll start with the MIT student database. While she learns that one of the victims was part of an animal rights group called Animals First, Astrid and the Bishops examine one of the bodies at the lab and find that, in addition to giant claws, the deadly creature had massive fangs. Walter looks troubled.
After another big animal attack in Newton, Charlie arrives on the scene first and finds two animal control officers dead with the same kind of wounds as the protesters ... then a giant tentacle-looking tail drops down behind him from the tree above. Olivia and the Bishops arrive just in time to hear gunshots from the woods. When they find Charlie, he’s hurt, but alive. He gets patched up by EMTs and released. A troubled Walter theorizes that the animal is made up of the genes of multiple species—“the best of the best, as it were,” he says. “Accelerated Darwinism.” Just then, Astrid calls: Her research shows that the Swift lab was the closest to the original crime scene.
At the facility, Dr. Swift stonewalls Olivia’s questions, so she returns to Walter’s lab. Walter is agitated and finally admits that he thinks that he created the giant hybrid creature 20 years before. But he’s confused: His experiments, done in concert with Kelvin Genetics, didn’t work. Astrid then notices one of the victims is moving in its body bag. Thinking the victim is still alive, they rush to unzip the bag ... and find larvae crawling around on the dead guy’s chest. Then, just when you thought it couldn’t get more gross, the man’s chest cavity bursts open and hundreds of larvae spill out. Walter deduces that the creature’s eggs are in its stinger. Olivia looks slightly ill. “Charlie,” she whispers.
At home, Charlie and his wife watch news reports of more animal attacks while they get ready for bed. (Geek note: If you look really closely, you can see The Observer in the background of the news report. Apparently, he’s in every episode!) Olivia shows up to tell him he’s not OK and accompany him to Walter’s lab, where an ultrasound shows that he has larvae swimming around inside him. Walter is going to try a poison to kill the larvae. He hopes it won’t kill Charlie. When it almost does, he says he needs some of the mother’s blood; he’ll inject it into Charlie’s blood in an attempt to fool the wee beasties into thinking he’s one of their own and not a source of food.
Olivia gets word that one of the MIT students’ last name was Swift. She confronts Dr. Swift about his missing son, Jonathan, and he breaks down while admitting everything. The creature came from his lab, a collaboration between him and a geneticist named Cameron Dagelman—the man killed alongside Jonathan. Dagelman was a pioneer in hybridization who inspired Walter’s work, but the beast had nothing to do with Walter’s former experiments. Oh, and by the way, the beast is part gila monster, part parasitic wasp, and part bat. Pretty.
After realizing that the creature is using the sewers to get around, Olivia and the Bishops go down to lure it out and procure its blood. Walter, still racked with guilt over being an "evil" scientist all those years ago, takes it upon himself to be live bait. It works; though the creature gets a good swipe at him, he shoots it dead. Back at the lab, the team gives Charlie the antidote and restores him to health in time to return him safely home without his wife any wiser. As Walter admits to Peter that he rarely considers consequences, Olivia goes home and sleeps with the lights on to keep the monsters away.
THE BOTTOM LINE: For the first time, Walter seems to realize that his playing God back in the day has really hurt people—so it’s ironic that this time, the mistake wasn’t his. In his lab notes, though, he seems, at turns, to hold himself responsible, writing:

I knew what I wanted. To turn Greek myth into modern reality. To create life like no one had seen before. To play at God.
As though that were a crime. God created more than a few monsters himself. Without men playing at God, we would still be huddled in caves with only skins to cover us. We have always bred beasts to make them useful to us—wild dogs into loyal pets, nomadic ungulates into dairy cows. Modern transgenics differs only in degree, not in kind, and
And in purpose? I didn't seek medical cures or better food or safer energy. I sought only to prove that I could, to find the limits of the possible—
To gain knowledge. The pursuit of truth, as noble a goal as any.