With a gaggle of onlookers, teenagers ďSky Commander WinkyĒ and ďCapín DareĒ are videotaping their latest stunts for the cable access ďDumb AssĒ show. Capín Dare (otherwise known as Bill) slips on a helmet, as Winky (David Winkle) pelts him with balls from a pitching machine. Capín Dareís girlfriend, Natalie Gordon, watches unhappily. So does another boy, Dylan Lokensgard. The boys move on to their next feat, and Capín Dare sits inside a portable toilet that is pulled by a car across a parking lot at a high speed. The car skids to a stop, and Capín Dare and the toilet roll down an embankment. Capín Dare emerges unharmed, and the crowd laughs. Natalie is again, not amused. They move onto their third stunt, where Capín Dare will roll down a hill in a shopping cart, onto a ramp that will sail him over Natalie, who waits on the ground. When the stunt goes awry, Capín Dare is thrown to the ground. Natalie and Winky are horrified to see that Capín Dareís head has collapsed inside his undamaged helmet.
Doggett and Reyes arrive at the Medical Examinerís Office in Ocean County, New Jersey. Dr. Fountain has requested help because he is unable to explain Billís cause of death. Suddenly, the corpseís eyelid flutters. Reyes gently opens the eye with forceps, revealing a hollow socket from which a swarm of flies emerge. They immediately call Scully in to make an assessment, and she concludes that the flies fed at such a furious rate that it caused the boyís head to collapse from within. The helmet actually protected his head in the crash. Yet Scully pronounces that no fly would act so aggressively. Dr. Rocky Bronzino, an entomologist who appears in the doorway, quickly refutes her conclusion. He informs the agents that the Australian bush fly and the New Zealand screwworm fly are both aggressive insects. However, these varieties are not the flies that apparently killed Bill. Reyes says that they are not investigating a murder by an insect, but a human.
At Garfield High School in nearby Manahawkin, New Jersey, Natalie sits in the cafeteria, saddened by her boyfriendís death. Dylan watches her from afar, as Winky approaches Natalie with a camera. He wants her to speak on the ďCapín Dare Memorial Video,Ē but she refuses, blaming him for making Bill do the stunts. Winky notices that Dylan has stood up to defend Natalie. He threatens Dylan, and spreads food on his head. Dylanís mother, an Administrator at the school, sees this and summons Winky to her office. Waiting for him are Reyes and Doggett, who believe that Winky may have killed his friend in order to sell the video to television networks. Winky denies the claim, and jumps out of his chair while violently scratching his back. Reyes and Doggett see that on his back is written DUMB ASS in red, bloody welts.
Dylan returns home, ignoring his mother to retreat to his bedroom. He lies on his bed, listening to Syd Barrett and looking longingly at a picture of Natalie. Mrs. Lokensgard knocks on his locked door, which makes him only raise his music louder. As he shuts his eyes to cry, bugs swarm over his window and begin to fill up his room.
The next morning, Dr. Bronzino informs the agents that the flies that ate Billís brain and skull happened to all be female. This suggests that something chemical or hormonal might have triggered the attack. Yet since lice had also caused Winkyís wounds, they wonder whether someone was somehow directing the bugs to act. They decide to question Dylan Lokensgard, who was present for every stunt and had a run-in with Winky just before the lice injured his back.
Dylan sees Natalie leave his front door. He rushes out the window, but his mother intercepts him and warns Dylan to stay away from Natalie. She tries to talk to him about the changes his body is going through, but Dylan brushes her off and heads for school. Meanwhile, Dr. Bronzino and Scully revisit the scene of Capín Dareís death, tracking the atmosphere with an electroantennogram device. While not-so-subtly hitting on Scully, Bronzino surmises that the bugs are somehow being driven crazy with desire. Suddenly, the deviceís signal loudly blares. They donít see Dylan, the cause of the deviceís alarm, peddling past them on his bike. Dylan finds Natalie at school, and she apologizes for Winkyís behavior in the cafeteria the day before. They reminisce about their childhood. Feeling uncomfortable, Natalie leaves abruptly. Reyes and Doggett accost Dylan, and question him in his motherís office. They show him the ďDumb AssĒ video, and he nervously begins to perspire. Doggett hands him a tissue to wipe his sweat. Mrs. Lokensgard enters, refusing to let their interrogation continue, when flies begin to gather on the office ceiling. Dylanís entire body is suddenly and mysteriously covered in flies.
A Vector Control team evacuates the school, but all the bugs have disappeared. The paramedics examine Dylan, who is unharmed with no bites on him. Reyes believes that Dylan staged the attack to make himself look like a victim instead of a perpetrator. Mrs. Lokensgard drags Dylan to the car, while Winky eyes him suspiciously. Dr. Bronzino examines the tissue that Dylan used to wipe his sweat, finding that the boy is actually secreting bug pheromones. Although that notion is preposterous, Reyes theorizes that Dylan is using the bugs to act out against anyone who might harm Natalie. Later that night, Dylan is surprised to see Natalie climb up to his bedroom window. She hasnít been there since they were in the fourth grade. She apologizes for not being a friend to him over the years, realizing only now how important he really is to her. He is ecstatic that his dream girl has opened her heart to him. They kiss, but she pulls away when Dylanís tongue causes her mouth to bleed. Natalie cries and quickly leaves, as Dylan tries to explain. He runs after her into the street, when another car stops behind him. It is Winky with a group of friends, who capture Dylan. They ask how he killed Capín Dare. Dylan responds angrily, ďI just have to open my mouth.Ē And when he does, a set of bug mandibles appears between his lips, spraying an opaque mist. The car loses control, turning upside down before it rolls to a stop.
Doggett and Reyes come upon the car wreck, where Winky and his friends are stuck under a thick spider web. He tells them that Dylan is the one responsible, and that he chewed his way out the back window. Reyes finds Natalie at home, curled up on her bed and crying. Reyes asks Natalie to talk to Dylan and convince him not to hurt anyone else. Yet Dylan is already standing in Natalieís bedroom. At the Lokensgard home, Scully and Dr. Bronzino arrive to question Dylan, but the house is empty and dark. They enter through the open front door, and Bronzino finds high readings of pheromone levels. After Scully leaves to join Doggett at the crime scene, Bronzino continues to scan the Lokensgard home with his electroantennogram. Mrs. Lokensgard surprises him, and she shoots a stream of webbing at Bronzino from the insect mandibles in her mouth. Dylan returns to his house with Natalie, and Mrs. Lokensgard confronts her son. He is upset when she tells him that he is not like the other kids and never will be.
Doggett arrives at Natalieís to find Reyes completely wrapped in layers of webbing, barely able to breathe. She tells Doggett that Dylan took Natalie to his house, which is where Scully is headed when she lost contact with Dr. Bronzino. Scully creeps in the Lokensgard home and sees Natalie sobbing. Scully cautiously goes into the attic with her weapon ready. Her flashlight beam locates a cocoon, from which comes the voice of Dr. Bronzino. Scully frees him. Paramedics arrive soon after, followed by Doggett and Reyes.
The agents discover that the other cocoon in the attic was a crude coffin for four bodies, one of whom was Dylanís father, long reported missing. Mrs. Lokensgard had killed her husband for not being what she and Dylan were -- neither human nor insect, but an anomaly in between. Although Dylan had tried to fit in with his peers and was in love with Natalie, he could not hide from his true nature. Mrs. Lokensgard escapes with her saddened son.
Natalie lies awake in her bed, when her attention is drawn to a light in the night sky. She goes to her window and sees a swarm of fireflies, their lights twinkling and forming a message that reads, ďI Love You.Ē