A classic! In my opinion, the best Xander episode ever. More of what I think later, now; the summary:
Buffy, Willow, Xander, and the rest of the school take a field trip to the zoo. The local bullies pick on Buffy, then a boy named Lance. The gang sees the bullies head into the hyena cage with Lance, and Xander goes in after them. Buffy and Willow try to follow them, but the zookeeper stops them and tells them that the hyena's are under quarantine. Meanwhile, inside; the bullies approach the hyena's and threaten to throw Lance in the cage. But, Xander comes to the rescue. Before they can do anything; the four bullies and Xander are suddenly possessed by the hyenas.
That evening, Xander joins Willow and Buffy at the Bronze; where he acts incredibly strange. He starts sniffing Buffy and takes amusement out of the bullies or "the pack" teasing others. Buffy and Willow are seriously wigged.
At school the next day, Principle Flutie introduces the new school mascot, Herbert the pig. At the same instance, Xander ditches Geometry tutoring with Willow. He throws his book in the trash and goes inside. As he's passing Buffy and the pig, Herbert squirms and tries to get away from Xander.
The overall Xander-creepiness continues when they play dodgeball in gym. Xander and the pack simultaneously attack the others with the ball, including Willow, until only Buffy and Lance are left. They all corner Lance and hit him repeatedly with the ball.
Afterwards, Willow approaches Xander and asks him to explain himself. He basically insults and rejects her while the pack laughs. Buffy steps up to him and challenges him, but he and the pack just walk off laughing.
Xander and the pack stalk around the schoolyard and scour for food, preferably raw. They finally find what they want inside, and his name is Herbert. So, they eat the school mascot. Then, they go outside and Xander sees Buffy and Willow talking. He listens to their conversation. Willow is afraid that she's done something, but Buffy knows better.
Buffy shares her concerns with Giles; who concludes that Xander has turned into a teenage boy. Buffy figures out that Xander has been weird since they went to the zoo, and guesses that it has do to with hyena's. Just as Buffy is pleading with Giles to believe her, Willow informs them that Herbert has been eaten. Giles goes to look stuff up.
Giles shows Buffy what will happen to Xander if they don't help him, and it's basic overall death. She runs to find him and looks where Herbert's cage is. Xander comes up behind her and she knocks him to the ground.
Principle Flutie finds the pack, not including Xander and takes them to his office. They corner him.
Buffy and Xander wrestle and Buffy tells him that he has to listen to her and they have to help him. He says that she likes her men mean, and now he's mean. She pushes him off of her and they square off. He shoves her and pins her against the vending machine.
The pack begins to trash Flutie's office and then attacks him. They basically eat him.
Buffy manages to knock Xander out and drags him to the library. Giles comes in and informs them of Principle Flutie's fate. He knows how to get rid of the possession, but he needs help. Buffy and Giles go to the zoo while Willow stays with Xander. He finally comes to and attempts to convince Willow to unlock the door to his cage. She sees through his act and won't.
The zoo-keeper explains that he can help them reverse-transpossess the students. They say they don't know where the whole pack is. He tells them not to worry, the pack will look for the missing member.
The other members of the pack break into the library and chase Willow. She hides in a classroom until she believes it is safe. But, when she comes out of hiding, Xander is there. She manages to escape and Buffy and Giles come to her rescue. They lock themselves in a closet until they leave. Willow and Giles go to the zoo while Buffy looks for the pack.
Buffy finds Xander and the pack attacking a family in their car. She stops them and "dares" them to chase her. So, Buffy starts running towards the zoo with the pack in tow.
Willow waits for Buffy to approach while Giles goes inside to find the zookeeper. Giles suddenly realizes that the zookeeper was trying to possess himself of the hyena before the students did. The zookeeper knocks him out.
Willow hears Buffy coming and rushes inside to inform the zookeeper. He ties her up and holds a knife to her throat. Buffy runs in and the pack attacks her. But, the zookeeper says a magic word and the possession enters him. Buffy battles him and throws him into the hyena cage, where he is eaten.
The next day at school, Xander claims memory loss and Willow and Buffy fill him in on his activities for the last few days. Except, they leave out some of the embarrassing details. The girls walk off and Giles informs Xander that in his research, he found no signs of memory loss. Xander becomes embarrassed, but Giles assures him that he won't tell anyone.