(Plot synopsis from the official Buffy website)
Buffy's kindly biology teacher, Dr. Gregory, is attacked after class by a giant preying mantis, but as far as the students know, their teacher is just missing. The mysterious Angel appears at the Bronze that night to warn Buffy about a pending attack. His arm looks as if it was cut by a tiger's claw.
The next day, Dr. Gregory's class is being taught by a substitute teacher the comely Miss Natalie French, who drives Xander ga-ga. She tells the class about the science fair, and asks for volunteers to help her create a model of praying mantis egg sacks after school. As expected, Xander's hand shoots up, as does football heartthrob Blayne's and every other boy in the class. Miss French decides that Blayne can be her assistant that afternoon, and Xander the next. Later in the cafeteria, Cordelia opens the fridge to discover Dr. Gregory's headless body. Buffy speculates that Angel's warning and his unusual wound have something to do with Dr. Gregory's gruesome murder, so she stakes out the park. She sees a vampire with one arm that has three sharp claws where the hand should be. The creature approaches Miss French, who is walking by with grocery bags, but instead of pouncing on her, he sniffs her and runs away in terror.
Buffy catches the teacher turning her head around all the way around. Buffy finally figures it out: Miss French is a praying mantis! Following her lead, Giles tries to get in touch with a friend in England who's an expert in the field the only catch being that the man is institutionalized. Meanwhile, hacker par excellence Willow has found that Blayne's mother has reported him missing. Since it's Xander's turn to work with Miss French this afternoon, Buffy warns him, but he thinks Buffy is just jealous. Miss French gives Xander a drink as soon as he shows up. As he's getting woozy, he thinks he hears someone calling, but she ignores it. She takes his hand, and her arm turns into a praying mantis' foreleg. He promptly passes out, and he is dragged away by Miss French. When he comes to, he finds himself in a cage in Miss French's cellar, with Blayne in an adjoining cage. Surrounded by headless bodies in cages, he calls to Miss French, who is now completely converted to her giant-mantis form.
After talking to his strait-jacketed friend, Giles confirms there is a creature called a She-Mantis, who lures male virgins to her nest. An extremely powerful foe, it was the She-Mantis who put Giles' friend into his deranged condition. Buffy suggests that Giles record some bat sonar from the video library playing that for the She-Mantis will disrupt her nervous system. Exhausting other methods, Buffy decides to enlist Claw Guy to help her sniff out the She-Mantis. Going from door to door, he finally winces when they reach the correct house. Turning on her, he tries to attack Buffy, who promptly kills him with a fence slat. Buffy enters the basement and uses a few blasts of the insecticide to send her scurrying off, then plays the bat sonar. The tape has the desired effect on the crazed insect, enabling Buffy to hack it to death with a machete. We think this is the end of the She-Mantis until we catch a glimpse of the inside of Dr. Gregory's classroom closet, where a glistening egg sack is stirring at the bottom.