On the night before Halloween, during a pumpkin-carving get-together at Xander's place, Buffy leaves the gang a little early to spend some time alone. Outside, Buffy knocks a demonic-looking figure to the ground. Still recovering from Parker's rejection, she barely reacts to the realization that it was merely a guy in his Halloween costume.

At school the next day, the sight of Parker drives Buffy out of the cafeteria just minutes into the lunch period. Willow tries to help out, but Buffy prefers the easy way out. Her growing preoccupation does not go unnoticed by Professor Walsh, who threatens to kick Buffy out of the class if she has any more absences. Riley suggests to Buffy that she should take a break from her worries and just have fun for Halloween. Meanwhile, Josh and Edward, two members of the Alpha Delt fraternity, prepare their house for a haunted party. While Josh puts the finishing touches on a knife-wielding skeleton, Edward hands over a bag of fake spiders, as well as a book that contains an appropriate symbol to add to the party's freaky ambience. When Josh complains about their stereo system's quality, Edward offers to call Oz to remedy the situation. After school, Oz brings his stereo over to the Alpha Delt house, with Xander tagging along. As Oz sets up the new sound system, Xander notices the symbol being painted on the floor of the attic by Chaz, another fraternity member. Oz accidentally cuts his finger while trying to tweak the sound quality by trimming the speaker wire. When he shakes some blood onto the floor, the painted symbol sends a rippling shockwave throughout the house that nobody notices. Suddenly, what was once a fake, plastic spider resting on the floor is now a living, walking creature.

Without a date of her own, Buffy feels left out as she accompanies the gang to the party. After being swayed from patrolling by Giles, Buffy wears an old Little Red Riding Hood costume, thanks to some last-minute alterations by Joyce. Willow embodies the persona of Joan of Arc in her medieval garb, while Oz completes the picture with his name tag reading "God". Sporting a James Bond-esque tuxedo, Xander tells the others that Anya, whom he invited earlier that day, will be a little late on account of costume-finding difficulty. The gang's stroll to the party is momentarily interrupted by the sudden appearance of masked commandos armed with rifles. They quickly turn and run, leaving the gang to mistake them for average Halloween celebrators. While Buffy and the others make their way to the fraternity house, the guests at the party soon realize that something is obviously not right. Bloody eyeballs, dead guests, and other horrific sights send the partygoers into chaos. In a rush to get away, Josh takes a fatal tumble down the main staircase, snapping his neck at the bottom step.

Moments later, Buffy, Xander, Willow, and Oz arrive. Real spiders and bats that appear to be fake at first quickly alert the gang to the trouble that lies ahead. Hidden inside a closet, they find Chaz huddled in the corner, too scared to come out. While trying to get any information from Chaz, Buffy fails to notice the skeleton that approaches from behind, the knife still in its hand. A slash at Buffy's back sends her into automatic pilot, flooring the enemy in one blow. Before it even hits the floor, the skeleton reverts to its plastic state. After inspecting the attacker, Buffy realizes that the closet that Chaz was hiding in is nowhere in sight. Having had enough, Buffy orders the others to evacuate and get Giles for help. Willow disagrees, believing she can conjure a guiding spell that will lead them to where they need to go. Their argument splits the group in two, with Oz following Willow while Xander tries to support Buffy. However, Xander soon realizes that he can no longer be seen or heard by Buffy, who starts a search for her seemingly lost friend. Oz, in the meantime, begins to transform into his wolf state, despite the fact that the full moon is still weeks away. Unable to control himself, Oz lashes out at Willow before running off to prevent any more harm.

Outside, Anya finally arrives in a big, white bunny costume. Unable to find the front door where it's supposed to be, Anya walks around the house and spots a terrified guest screaming through one of the windows. As Anya looks on, the window suddenly vanishes from the facade. She immediately seeks out Giles, who goes through his books to find answers. After locating what he needs, Anya returns to the house with Giles, who employs a chain saw to get them inside. Meanwhile, Willow attempts the guiding spell. The appearance of a tiny, green sprite leads Willow to believe she succeeded. However, the sprite rapidly multiplies into an uncontrollable swarm that chases Willow from room to room. Elsewhere, a door mysteriously lands Buffy in the basement, where she finds herself restrained on the floor by the arms of zombies. As she struggles to break free, Josh appears and taunts her mercilessly. When Buffy finally escapes, she ends up in the attic, where the rest of the party guests shiver in fear. She finds Oz, who has returned to his human state. Willow arrives shortly, followed by Buffy's sighting of Xander in another corner of the room.

Back together, the gang tries to make sense of what's going on. Xander points out the symbol on the floor, which Willow identifies as the Mark of Gachnar, a fear demon. They conclude that their fears are being manifested so that Gachnar can enter their reality. Suddenly, a buzzing chain saw marks Giles and Anya's entrance into the attic. Giles produces the book he found on Gachnar, which describes several ways of preventing the summoning. Halfway through Giles' reading of a sentence involving the destruction of the symbol, Buffy sends her fist through the Mark of Gachnar. Unfortunately, the rest of Giles' reading informs Buffy that she has just brought forth Gachnar. A bright light shines through the hole in the floor, and the gang watches as Gachnar emerges... as a diminutive demon no taller than several inches. While suppressing her own laughter, Buffy puts a quick end to Gachnar with one stomp.

~Episode Summary by: Alan Hufana