We begin this episode with a wet dream courtesy of Xander where Buffy is falling all over him. Xander saves her from a vampire then gets up on stage at the Bronze (the local club) and starts rocking guitar. Fortunately we find out that he was just asleep in science class. They're studying the insect world. Buffy gets in trouble and has to stay after class, where the proff does inspirational speaking and we establish that Buffy likes him as a teacher. Buffy leaves and the prof. gets gacked by...well, the title tells us...a giant preying mantis.
Cut to the Bronze where Angel delivers a cryptic message about 'he's coming'. The next day Buffy finds out her favorite teacher is missing, and Xander practically orgasms when he sees the new substitute teacher. She does a rather obvious angry twitch when Buffy calls the preying mantis butt ugly. Please, could we make this anymore obvious? And while she's talking about the female mantis choosing a mate and then EATING HIM, she's staring at Xander suggestively. Xander is smitten, as are all the other males in the class, so much so that they all volunteer to help the sub make egg sacks after class. No I'm not joking. Buffy and Willow are baffled.
At lunch the good teacher's headless body is found in a freezer. The group is understandably shocked. That night Buffy goes on patrol for a possible lead, that being the cryptic 'he' referenced by Angel. This Vampire with a giant claw tries to run, and is about to assault the new sub teacher while she's out walking. She just looks at him and runs for the hills.
Buffy tries to get to class when she's cornered by the Principal demanding she get couseling for seeing the teacher's body yesterday. She is therefore late for class and witnesses the sub's head doing a 180 to look at her. Buffy realizes that some bugs can do this and goes to research bugs. Meanwhile Xander gets invited to the sub's house that night and he is quite excited. Buffy and the gang realize that the sub is a preying mantis and go to save Xander.
The sub DRUGS XANDER with a dirty martini and takes him to her lair. Buffy breaks in just as the sub is about to mate with Xander, kicks butt, and Xander destroys the egg sacks. We get a flash of an egg sack they missed at the end of the episode, but I think they forgot about that thing to bring back later as it hasn't been mentioned again.