While 0n a field trip to the Sunnydale Zoo, Buffy, Willow, and Xander watch as bullies pick on Lance, a school nerd. When the bullies take Lance into the new and closed hyena exhibit, Buffy sees a problem. Xander steps up and offers to handle it. "This job doesn't require actual slaying," he tells her. When he reaches the bullies, they are teasing the kid, threatening to throw him in with the hyenas. Xander rushes up to help. "Why don't you pick on someone your own species?" he asks Kyle, the leader of the pack. All of a sudden, everyone's eyes turn yellow, including Xander but not the weak one. Lance turns to run and falls, as the group bursts into hysterical, hyena-like laughter.
Later, Willow and Buffy notice that Xander is behaving strangely; cruel and aggressive. He insults Willow and blows off his studies. In PE during a game of dodge ball, Xander deliberately throws a ball at Willow and it hits her hard. When Willow confronts him, he says something hurtful and then bursts into hysterical laughter. He leaves with his new friends, the pack of bullies, and proceeds to wreak havoc in the school.
Buffy, worried something Hellmouthy is happening, goes to Giles for help. He blows her off, telling her it's normal for adolescent males to have mood swings. "Testosterone is a great equalizer," he says. "It turns all men into morons."
But when Buffy learns that Sunnydale's mascot, Herbert the pig, has been eaten, Giles is forced to believe her. Giles remembers something he has read about animal trans-possession. They realize that Xander and the others have been possessed by the spirits of the hyenas. Worried for Xander's safety, Buffy goes to investigate and finds him by Herbert's mangled cage. They fight and Buffy ends up subduing him. Right after, she locks him in the cage in the library.
Meanwhile, Principal Flutie is lecturing the bullies in his office. While telling them how deeply disturbed he is by their murderous behavior, they attack him. Soon, everyone learns that Principal Flutie is dead. Willow is thankful that Xander was with Buffy when Flutie died.
Buffy and Giles go to the zoo to talk to the zoo keeper about how to reverse the possession, and return the hyena's essence back to their rightful bodies. The zookeeper tells them that they will need to get all the possessed back to the exhibit so he can perform the reverse trans-possession.
Willow is watching over Xander when the bullies come for him. They chase her, but she manages to escape, thanks to Buffy, who distracts them. They rush after her and she leads them back to the zoo.
Meanwhile Giles and Willow head for the zoo. Willow waits outside the exhibit while Giles heads in to prepare for the ritual. Inside the exhibit, Giles realizes that the zoo keeper himself, unleashed the hyena's spirits, but he messed up. His intent was to bring the hyena's essence into himself and was angered when he learned that it possessed the teenagers instead. "How terribly frustrating for you, that a bunch of schoolchildren could accomplish what you could not," Giles says to him. "It bothered me," admits the zoo keeper, "but the power will be mine." He knocks Giles out with a blow to the head and drags him off to the side.
Outside Willow hears the others approaching and runs inside to warn Giles.
The zoo keeper grabs Willow, ties her arms and puts a knife to her throat, just as Buffy and the others arrive. He says the spell that will force the spirits from the teenager's bodies and into himself. Xander, who is back to normal, gets Willow out of harm's way, while Buffy deals with the zoo keeper. The zoo keeper lunges at Buffy, but she is prepared and she throws him off. He lands in the hyena pit, and the beasts tear him to pieces. Moments later, Giles rushes into the room.
The next day at school, Buffy, Willow and Xander discuss Xander's animal behavior. Xander seems to have forgotten everything that happened and is shocked to hear that he ate a live pig. "Well, I remember I was going on the field trip and then down in the hyena house and then next thing some guy's holding Willow and he's got a knife." Buffy and Willow assure him that nothing else happened, letting him off the hook for all of his horrible behavior. Buffy and Willow head off to happy to have Xander back to normal.
Moments later, Giles confronts Xander with the news that he cannot find anything in his books to indicate memory loss after animal possession. "Did you tell them that?" Xander asks, pointing towards Buffy and Willow's retreating figures. Giles assures him that his secret is safe, and Xander walks away, mortified and relieved all at the same time.