A vampire is menacing club-goers at the Bronze, but Buffy, dressed in a sexy red outfit, is unable to fight him off. Just as the vampire is about to bite her, a cooler, more suave Xander rushes in to save the day. With a quip and a yank, he pulls the vampire off Buffy and throws a stake into its chest. Buffy watches him in awe, adoration shining on her face. After promising to kiss her like she's never been kissed before, Xander jumps onto the stage and plays a sexy guitar riff. Buffy smiles up at him. "You're drooling," she says. Xander is confused for a second, and then his eyes jerk open. There is no vampire, no guitar, and no Buffy in a red dress. Nope – it was all a dream, and unfortunately for Xander, his less pleasant reality is science class at Sunnydale High.
In Dr. Gregory is showing the slideshow on ants. He calls on Buffy to answer a question, but it is clear she doesn't know the answer. He pulls Buffy aside after and encourages her to apply herself, because he knows that she can excel in his Unused to positive criticism, Buffy is both flattered and inspired. She agrees to work harder and leaves the room. Dr. Gregory, busy examining some of his slides, doesn't hear the door of his closet creak open. Unaware that he is not alone, he continues his work. Suddenly a green tentacle-like claw reaches out and pulls the teacher off the chair. Dr. Gregory groans and falls to the floor, his glasses shattering on the floor.
At the Bronze that evening, Xander has a run-in with Blayne, who is bragging about how many girls he's slept with. When Blayne asks Xander how many times he has scored, Xander feels his manhood is in question. Xander bluffs a moment, and spots Buffy and Willow. He walks over to them, and showing off, puts an arm around each of their shoulders. Willow snuggles right up. "We'll show him," she says, pleased with the opportunity to cuddle Xander. But Buffy is distracted by the sudden appearance of Angel. Ever the cryptic, he warns her about a coming danger and drapes his leather jacket around her shoulders. She notices his arm is hurt – there are three slashes across it, almost like the prongs of a fork. But Angel is vague (shocker!) and dodges the question. Without further explanation, he leaves, disappearing into the night.
The next morning, Buffy tells Giles about Angel's cryptic warning. Giles promises to look into it. As he turns to go, Xander approaches Buffy and Willow who are sitting on the bench outside the school. Xander is excited to share the news that Dr. Gregory won't be coming in today – which means that he can't get in trouble for skipping his homework. Concern hits when Xander reveals that the teacher isn't sick – he's missing. Buffy is worried, and Xander starts to console her, when distraction comes in the form of a beautiful, long legged woman. The raven-haired beauty asks Xander if he can direct her to the science room. He stumbles over his words, frustrated with his inability to recall the quickest route, even though he goes to that room daily. Blayne sees this as an opportunity to both humiliate Xander, and talk to a pretty woman, so he swoops in, and offers to take her to the room.
In the room, Buffy notices that Dr. Gregory left his glasses on the floor. She picks them up and places them on the desk. "If Dr. Gregory dropped his glasses," she wonders, "why wouldn't he pick them up?" Shrugging off her worry, Buffy takes her place at her desk, waiting for the lesson to begin. The substitute teacher is none other than Natalie French, the mysterious, raven-haired beauty. Unlike most substitutes, Ms. French is clearly knowledgeable in the subject. She starts the off with a discussion on the praying mantis. When Buffy calls the mantis "bug ugly" she tenses visibly, and defends the inherent beauty of the insect. Ms. French suggests that the make model egg sacs for the school science fair and asks for volunteers. The hand of every male in the room shoots up, eager to spend time with the young, attractive teacher.
In the lunch line, Blayne approaches Xander, mocking him that Ms. French asked Blayne to help him first, while Xander will have to wait until the next day for his turn. As Xander fires off a retort, Cordelia pushes through the line to the back, explaining to the lunch lady that she is there to pick up a medically prescribed lunch, shipped daily by her doctor. "I'll only be here as long as I can hold my breath," she assures her. Cordelia opens up the fridge and lets out a terrified scream. "His head! His head! Oh my God, where's his head?!" Cordy's scream attracts the attention of Buffy, Willow and Xander who come running to see what Cordelia has already seen – the headless body of Dr. Gregory stuffed inside the refrigerator.
Later, in the library, Buffy, Willow, and Xander tell Giles about Dr. Gregory's fate. Buffy is clearly distraught, tears staining her cheeks, as she tries to comprehend why this happened. Buffy wonders if Dr. Gregory was killed by "Fork Guy" – the threat that Angel warned her about. Giles tells her that Fork Guy is not the likeliest suspect for Gregory's death, as his method of killing is not decapitation, but rather shredding and exsanguination. Giles states that the clawed vampire is a follower of The Master, whose hand was cut off in penance for a failed Mission. Buffy wants to patrol, but Giles says that more research is needed. Against his warning, Buffy decides to investigate.
In Weatherly Park, Buffy finds the bladed vampire. They fight and just as she brings the stake down, he rolls out of harm's way. He jumps up, preparing for more but is startled when the police come. He runs off into the woods and Buffy follows. She sees Ms. French walking alone, bags of groceries in her arms. Fork Guy stalks her, but before he can attack, Ms. French turns and stares into the vampire's face. He backs away slowly, and dashes for the sewers, terrified. Buffy, witnessing all of this, stares as waves of shock roll through her.
The next morning Giles is disappointed in Buffy for flagrantly disregarding his orders to stay inside and avoid patrolling. But he gets over it quickly when she tells him of the vampire's weird response to Ms. French. Giles instructs Buffy to keep a close on eye on the substitute teacher. She agrees and rushes off to science on the way; she is waylaid by Principal Flutie who wants anyone who saw the body, to attend grief therapy. Buffy tries to argue, but he is firm as he pushes her into chair to wait. When she finally arrives too late, she peers through the window and sees the taking a quiz. She looks at Miss French and the teacher stares back, her head turning a full 180 degrees. Shocked, Buffy quickly slides out of view and heads right back to the library. She tells Willow and Giles of the "full on exorcist twist" that Ms. French performed. They discuss what possible creature could do such an abnormal feat. Buffy researches and realizes that only the praying mantis can twist its head in that direction.
Xander comes to see Ms. French who is fixing her lunch. She flirtatiously tells Xander that she forgot the materials at home and doesn't have time to retrieve them. Is it possible that Xander could come to her home to work on the project? Xander is ecstatic and eagerly agrees. After he leaves, Ms. French finishes preparing her sandwich – live bugs and mayonnaise. She takes a bite, smiling in satisfaction.
On the computer, Willow discovers that Blayne, who worked with Ms. French yesterday, never came home the night before and didn't appear in school today. Fearing for her friend's safety, Buffy rushes off to locate Xander. When she finds him, she tries to delicately warn him that Ms. French is not what he thinks. He doesn't get it, so she finally comes out with it: Ms. French is a big bug. Xander laughs at her, and accuses her of being jealous. "She's not an insect! She's a woman," he says angrily, "And hard as that may be for you to conceive, an actual woman finds me attractive." He rushes off, leaving Buffy staring after him in bewilderment.
Elsewhere, an elegant hand is pouring martinis. Ms. French is dressed to kill in a very sexy and revealing dress, her makeup done to devastating perfection. Xander arrives, staring at Ms. French's ample chest like a love struck school boy. She offers him a drink and he accepts it, downing the martini in a single, unsophisticated gulp. Ms. French just giggles at him seductively. She strokes his face and hair, and asks him if he has ever been with a woman. He stumbles over the answer, attempting to lie, before finally admitting that he is still very much a virgin. He hears a noise, but she distracts him, asking him if he would like to touch her. He stares at her creamy slender hands, and starts to compliment them, when suddenly they transform into the very same green tentacles that attacked Dr. Gregory. He blinks and then falls unconscious: the martini, it seems, was drugged. Two mantis claws appear and drag Xander's body away.
When Xander awakens, he finds himself in a basement, locked in a cage. He sees a giant mantis kneeling over something. "Ms. French?" he says uncertainly. The bug turns to look at him. "Please," it says, "call me Natalie." Xander stares in shock. He backs away towards the back of the cage and notices Blayne in the cage next to him. Blayne is distraught and terrified. He tells Xander what is in store for them. Ms. French, in mantis form, will take them out of their cages, mate with them, and then bite their head off. "I don't want to die like that man," Blayne cries. Xander tries to calm him, as he frantically thinks of a plan. Unfortunately, there doesn't seem to be any way out for the two virgins.
Meanwhile, Buffy, Willow and Giles are in the library, frantically trying to find information on Ms. French. Giles confirms that Ms. French is indeed a giant bug, and that she lures innocent virgins to her nest to mate with them. Willow tries to contact Xander but learns from his mother that he has gone to a teacher's house to work on a project. Buffy takes charge. She orders Giles to go to the video library and record bat sonar-the sound will destroy the nervous system of the She-Mantis. She turns to Willow and asks her to check the school's substitute rolls and get Ms. French's home address. Meanwhile Buffy will load up on armory.
Back in the basement, Xander removes one of the bars separating Blayne's cage from his own. Blayne is at first excited, but he quickly deflates when he realizes that he might be able to leave his cage, but only to enter Xander's. He brandishes his makeshift weapon as the She-Mantis turns his way. Blayne cowers, and Xander backs as far up against the wall as possible.
Having loaded up on weapons, Buffy returns to the school library. Willow has retrieved the teachers address and points out that her records indicate a birth year of 1907 – the teacher must be 90 years old. Giles tells Buffy that he has the requested bat sonar on tape. The group is ready to roll and they hurry from the library to find Xander and slay the She-Mantis.
Meanwhile, in the basement, the She-Mantis has lured Xander from his cage. Xander comes out slowly, his weapon hidden behind his back. Just as he finally gets out, he swipes the weapon at the bug's face and runs for the stairs. She is too quick though, and trips him just as he reaches the top. He falls to the bottom, too dazed and bruised to stand and run.
Buffy arrives at Ms. French's house and prepares to knock down the door with a well-placed side kick. But before her foot can make contact, the door opens to reveal a sweet old lady-the real Natalie French. Buffy realizes that the She-Mantis must have used the real Ms. French's information to get the substitute position. "She could be anywhere," Buffy says in frustration. Buffy is sure that the bug lady must live in this neighborhood, because it is in the same area that she saw her carrying groceries. Willow decides to start banging on doors but Buffy thinks that idea is impractical. "We have to do something," Willow cries. Buffy agrees and leaps into the sewers. "I won't be long," she promises. Minutes later, she reappears, pushing the tied up, bladed vampire in front of her. She uses his fear of the She-Mantis to sniff out its home: when the vampire becomes frightened, she knows she has found the correct house. Finally, she dusts the vampire and prepares to save Xander.
The mantis is just about to kiss Xander when Buffy comes crashing through the windows. Armed with two cans of bug killer, she sprays them directly into the eyes of the She-Mantis. Giles and Willow rush to rescue Xander and Blayne as Buffy fights with Ms. French. She reaches for the tape recorder with the deadly bat sonar and presses the play button. Unfortunately, the tape is on the wrong side and only Giles's voice can be heard. The bug attacks and the tape recorder is knocked across the room. Buffy and the mantis spar and Xander rushes into help, the can of bug spray pointed squarely at Ms. French's face. His efforts are foiled when Buffy pushes him squarely out of the way. Just then Giles reaches the tape recorder and the bat sonar plays loudly into the room. While Ms. French's nervous system goes crazy, Buffy chops at her with the machete. Finally the beast is dead. Xander is apologetic. "Just for the record, you were right. I'm an idiot, and God bless you," he says to Buffy.
Willow embarrasses the guys when she tells them how unfair she thinks it is that the mantis only went after virgins. Blayne tries to deny it, but Giles interrupts. "That's the She-Mantis operandi," he says. "She only preys on the pure." Blayne, mortified at being exposed, threatens to sue anyone who reveals his secret. Xander, also embarrassed, picks up Buffy's machete and takes his aggression out on the large egg sacks hanging from the ceiling.
The next day in science there is a new teacher and things seem to be back to normal. But this is Sunnydale and nothing is normal. Because hidden in the science room closet, there are a bunch of giant eggs…and one is starting to hatch.