A young, attractive, professional woman boards the bus along with a suspiciously large number of people. Something isn't quite right, and she's a little off put on the unusually crowded ride, sandwiched between strangers. But things take on a lighter mood when one of the passengers pulls out a boom box and starts blasting the YMCA, as all of the passengers join in. It's a flash mob; just some harmless fun! The jovial crowd gets off at the next stop, only leaving the bus driver to find that the young woman is left on the floor of the bus, dead.
Nick and Hank arrive on the scene to investigate. The young woman was a lawyer named Serena, and she seems to have suffered a puncture wound. Whatever was injected left her body swollen and disfigured; there's definitely some foul play at hand. The killer's motivation, however, is a complete mystery.
They take their body to the expert, Harper, for further analysis. The victim went into shock from an overdose of apitoxin, also known as bee venom. However, the mass amount that was injected is seemingly impossible to harvest from bees.
Nick, Hank and Sgt. Wu analyze the security camera to try and get some insight. But the mob blocks the action; it could have been anyone. One thing's for sure - this wasn't some random killing. The flash mob is the witness pool; it's time to call them in to the station.
Hank and Nick go to question the local bee expert, Professor Spinella. The professor analyzes their bee venom sample and reveals that it is not synthetic apitoxin. But it is untraceable and has no known origin. The bee expert himself is stumped; it looks like Nick and Hank are out of luck.
The flash mob arrives at the station to be interrogated, and it seems that everyone has the exact same story, in fact, suspiciously so. They got the invite on twitter to meet at the bus station at the particular time and dance to the YMCA as part of a flash mob. Everyone claims to be strangers, and no one claims to have seen the victim on the bus. A particular witness, Doug, seems to be more on edge than others. When Nick catches him in a lie and asks to see the tweet in question, Doug suspiciously refuses to show it. When Hank reaches for the phone, Doug grabs it, his arm morphing into that of a bee. Nick later sees another witness in the waiting room morph into a bee creature as well. Something supernatural is definitely afoot.
Nick and Hank tail Doug and another witness (who previously claimed to be strangers) to an old paper mill. They witness the two men meeting an unidentifiable woman. Before they can get any closer, Hank and Nick get attacked by a swarm of bees and are forced to make a run for it. What on earth is going on?
Back at Nick's place, Juliette treats Hank's stings and tells him to rest up. While Hank rests, Nick refuses to let up; he needs to learn the identity of the woman at the paper mill. Juliette comments on Nick's constant absence, noting that he has been turning this no sleep thing into an art form.
Later, Nick's back in his aunt's trailer, and his suspicions are confirmed when he finds bee artwork in his aunt's old reference book. The creatures are called Mellifers and seem to be dictated by the Queen Bee. Nick calls Monroe for help and meets him at the paper mill to follow up on his lead. Monroe explains to Nick that Mellifers are gossips and run the creature community switchboard. When Nick questions why a Mellifer would murder an innocent woman, Monroe remarks that maybe she wasn't so innocent. Mellifers have only one natural enemy, the Hexenbiest.
Monroe follows his nose to the paper mill office, leading Nick to uncover the identity of the woman in question: Melissa Wincroft. They follow the paper trail to her address and arrive at her mansion shortly after. The two break in to the apparently abandoned house to discover dead bees and honey coating the floor. When they brave the mess and climb upstairs, they find that the house is a giant beehive, coated with enormous beehives.
The next day the police force arrives at the scene. The apitoxin created by the bees in-house is the same used in the murder of Serena, making Melissa Wincroft the primary suspect. Nick uncovers the connection between Serena and Melissa; Serena was one of three lawyers on a team who shut down her paper mill company, so it looks like Melissa was out for her revenge.
Nick and Hank get a call for another flash mob murder. When they arrive at the scene, they find another dead lawyer from the same firm; this time the mob did the hokey pokey. Nick has a sudden realization; there was a third lawyer on the case, Adalind Shade. Concerned for her safety, Nick makes a mad dash to get ahold of her before it's too late.
Nick instantly recognizes her as the woman from the hospital who tried to murder his aunt and remembers her transformation into the gruesome Hexenbiest creature. While the two pretend to not know each other, Nick is clearly frazzled. Nick wonders: is there a connection other than the law firm between these three victims? After investigating the dead bodies in the morgue, he finds a mark under the tongue of the deceased, which he has learned to be the mark of the Hexenbiest from his aunt's research.
Nick's behavior towards Adalind continues to be rude and insensitive, to Hank's confusion. Later, Adalind tells the captain (who we know has been conspiring with Adalind) that she doesn't trust Nick to protect her since he remembers her identity. It's of the utmost importance that he finds the queen; if he doesn't, she's not the only one who's going to get stung.
The squad sets a plan in motion to bring Melissa out of hiding. Nick again questions Doug and at one point leaves him in the interrogation room with his phone as bait; a Mellifer can't resist this kind of temptation. Doug takes it and sends a text telling Melissa to make the move and attack Adalind. This is just what the team had hoped for, and they're prepared. They send a decoy Adalind into the law firm, while holding the real Adalind in protective custody.
Unfortunately, this decoy is no match for Melissa's cunning abilities. The queen stages an attack, releasing bees from her car and sending them in to their place of hiding. In the commotion of the bees taking over the room, Adalind makes a run for it, losing Hank and Nick.
Hank and Nick split up to find their potential victim; Hank runs up the stairs while Nick makes his way to the basement. Nick arrives in the basement to find none other than Melissa Wincroft, armed with a syringe of apitoxin. While Nick has his gun drawn, Melissa explains that the Hexenbiests are the bad guys; she's on his side. She has no choice but to kill Adalind, as the lawsuit was a conspiracy to get rid of Mellifers for good.
Adalind walks in on the scene, and a fight to the death breaks out between her and Melissa. With his gun still drawn, Nick is left with a serious dilemma: whom does he shoot? When Hank arrives on the scene, Nick knows he has to make a decision of whom to take down. Nick shoots Melissa twice, opting for his duty as a cop over his duty as a Grimm. Her dying words to Nick leave him with even more questions: "He's coming for you. Beware, he's close."