She'll sting you one day. Oh, ever so gently, so you hardly even feel it. 'Til you fall dead.
A woman, Serena Dunbrook, boards a streetcar and stands with the other passengers. A teenager reaches into his duffel bag and takes out a metal canister, and... starts playing YMCA. The passengers form a flash-mob and dance along until the next stop, when everyone gets off except for Serena. When the driver goes back to investigate, he discovers that she's lying on the floor, dead. Nick and Hank are called to the crime scene, where Sgt. Wu has identified the body and confirmed she works for a prestigious law firm, Berman, Rauthbert & Associates. When the detectives examine Serena's body, they discover that her body is grotesquely swollen due to anaphylactic shock.
At the morgue, Dr. Harper confirms that Serena died of anaphylactic shock and had 500 times the amount of bee venom in her body that a single bee could inject. The coroner also confirms that the puncture wound on Serena's neck was made by an abnormally large injector, and that Serena wasn't allergic to bee venom. The detectives go over the security footage of the flash-mob without success, as they block the view of Serena after she collapses. Renard tells them to close the case to avoid public panic over public transportation and sends them to talk to Harrison Berman, Serena's senior partner. Berman and Serena's co-worker, Camilla Gotleib, are both upset at her death and offer to do whatever they can to help.
Harper directs Hank and Nick to Professor Elliot Spinella, a beekeeping expert. He confirms that the bee venom that killed Serena is natural, not synthetic, but is unable to determine the point of origin. When they wonder if he might have been responsible, Spinella provides an alibi and notes that he was at a convention in Eugene at the time of the murder.
When the officers return to the station, they discover that members of the flash-mob have responded to the television coverage and come in to testify. They all claim that they received an anonymous tweet inviting them to the streetcar to dance. However, when Nick interrogates one of the suspects, Doug Shellow, Doug contradicts himself and refuses to let the detective sees his phone. Nick is able to see Doug's true face, a bee-like creature. When he meets with Hank outside to confirm that the tweet was sent through an anonymous, he sees another witness, John Coleman, who is also a disguised bee-creature.
The partners follow Doug to Primrose Paper Mill, an abandoned plant. When they go inside, they spot Doug meeting with another flash-mob member and a brunette woman. The detectives are unable to get a good look at their face. Before they can move in, a single bee lands nearby and then a swarm appears to attack them. Hank and Nick take refuge in an office and seal the doors, and the bees immediately leave. As they go to get Hank medical attention for his stings, Nick notices white shoeprints on the floor.
At Nick's house, Juliette removes the stingers from Hank's arm and tells him to get some rest. Once he leaves, Nick tells Juliette that he has to do some research and she points out that he's spending a lot of late nights doing "research." He goes to the storage yard where he's keeping Marie's trailer and reads his aunt's notes. There are pictures of the bee-creatures, mellifers, attacking a hexenbiest, the creature that attacked him at the hospital and poses as Adalind Schade. There's also an entry on how the hexenbiests have a mark on their tongue no matter what their form he's in. As Nick works, he's unaware that a bee is at the window, watching him.
Nick calls Eddy and they rendezvous at the paper mill. As they go in, Eddy explains that the mellifers relay messages and typically aren't killers unless threatened. Nick has him follow the woman's scent to an office where they find an envelope confirming she's Melissa Wincroft, owner and CEO of the paper mill. They go to her home and discover that she is keeping beehives in her attic.
Nick calls in the police to secure the scene, and tells Hank that he checked the legal files. Serena represented a company conducting a hostile takeover of Primrose Paper, and Doug and Coleman unsuccessfully fought it. They figure that Melissa is out for revenge for losing her family-owned business. The partners are called to the park where Camilla has turned up dead, the victim of another flash-mob murder. Like Serena, Camilla is dead of anaphylactic shock. She worked the Primrose case with Serena, and Nick realizes that there was a third attorney involved. He gets the attorney taken to the station and put into protective custody, but discovers that it's Adalind, the hexenbiest. He's momentarily shocked and Renard talks to him privately, asking if he knows the woman. Nick lies and says that he doesn't, and Renard tells him to focus on the case and tell him anything that he does know.
Realizing the pattern, Nick goes to the morgue and confirms that the two dead women have the mark of the hexenbiest on their tongues. He then goes to talk to Adalind and asks why Melissa is taking the case so personally. When Nick continues to interrogate her, Hank takes him outside and tells him to back off. Meanwhile, Renard meets secretly with Adalind and assures her that he'll protect her, and she tells him to find Melissa.
When the detectives are unable to pick up Melissa's trail, Nick suggests that they get Doug to lead them to her. He starts to interrogate Doug again and Hank calls him out of the room. As Nick goes, he leaves Doug's telephone on the desk. The cops watch from the next room as Doug finally calls to tell Melissa that the police don't know that Adalind is the target. They send an undercover agent disguised as Adalind to her apartment while Hank and Nick take the real Adalind to a hotel. Nick continues to harass Adalind, and once they're alone, he tells her that he knows who she is. She refuses to tell him anything and Nick insists that he needs to do his duty. However, if he doesn't get answers, he might have a bad day at work.
Melissa calls in Spinella, a mellifer, and the professor is reluctant to blow his cover. She forces him to help her, saying that "he" will understand that she's doing what needs to be done. Melissa then unleashes a swarm of bees, which swarm over the hotel. One of them spots Hank in the bathroom and summons the others. Meanwhile, Nick warns Adalind if she doesn't tell him everything, and she warns him that he doesn't know what he's getting involved in. When the detective threatens to let her hang if she doesn't cooperate, Adalind reminds him that it won't just be a Grimm killing a monster, but a cop sacrificing an innocent woman.
Before Nick can pursue the argument further, he hears Hank yell for help and runs into the bathroom. They try to drive off the bees, but the insects suddenly withdraw on their own. The partners realize that Adalind has fled the hotel room and split up to find her. Nick goes down the stairs and spots her entering the boiler room, and goes in after her. He discovers that all of the bees have gathered in the room, and Melissa is there armed with a syringe filled with bee venom. She tells Nick that she's trying to help him by killing the hexenbiest on his behalf, because something bad is coming. Melissa then finds Adalind and the women fight. Hank arrives and goes to Adalind's side, unaware of her true nature. When Melissa tries to attack him, Nick shoots her. Dying, Melissa staggers toward Nick and says that "he" is coming for the Grimm. When Melissa dies, all of the bees in the room die with her.
Later, Nick is in bed with Juliette and remembering what his aunt told him about killing the bad monsters. She tries to comfort him, saying that he made a difference by saving Hank and Adalind, and he admits that he feels a little better. Nick then goes to the window to close it and sees a bee hovering outside. It lands on his hand... and stings him.