Things begin with Buffy, Willow and Dawn clearing the house of Willow's magic items. Meanwhile, across town, the nerd trio evaluate their invisibility ray. Back at Buffy's house, Dawn is mad at Buffy and Willow. Out of nowhere, Spike enters the house, looking for his lighter. Buffy thinks that he is making lame excuses to see her. After a short exchange between Spike and Buffy about her hair, Xander interrupts, reminding Spike that he doesn't have a chance with the Slayer. As Dawn leaves for school with Xander, an unfamiliar woman is at the door. She is Ms. Kroeger, a social worker. After a brief conversation that makes Buffy look like a horrible guardian for Dawn, the social worker leaves, telling Buffy that she is on probation and will be monitored closely. Angry, the Slayer takes out her angst on her hair, chopping it off. Immediately, we find Buffy at a local hair salon getting the mess fixed.
Afterwards, Buffy exits to the street where the Super Villains are playing with their ray gun. Things get messy when Jonathan and Andrew fight over the gun causing it to misfire, accidentally hitting Buffy and making her invisible.
At the Magic Box, Xander and Anya are bickering about their wedding. To their surprise, Buffy interrupts their conversation and announces that she's invisible. Xander decides to look for clues and Buffy decides to go for a walk. Xander and Anya conclude that Buffy's state is the result of one of Willow's botched spells. However, when Xander finds Willow, he realizes that she is not to blame.
Enjoying her new invisibility, Buffy creates havoc on the streets of Sunnydale. She makes fun of a woman with a studded hat, picks on a bully, then snags a Meter Man's scooter and heads to the Social Services office to settle the score with Ms. Kroeger. With the help of her invisibility, Buffy makes Kroeger look like a crackpot to her fellow workers. The end result: Buffy gets a second chance as Dawn's guardian with a new social worker.
Downtown, Xander and Willow look for clues. They come across an invisible fire hydrant that has paint scrapings from Buffy's mystery van on it.
At Spike's crypt, the bleached vamp is shaken by an invisible guest. To his surprise, it's Buffy. Meanwhile, at the Magic Box, Anya and Xander make a horrible discovery when their tests reveal that a pylon hit by the invisibility ray is turning into pudding. They fear Buffy may suffer the same fate. That fear is confirmed as we join the bickering trio at their lair, discussing how much radiation Buffy absorbed when the ray accidentally hit her.
Xander enters Spike's crypt, desperately trying to find Buffy. To his shock, he finds Spike near-naked, "exercising" on his bed. The truth is that Spike is getting a little naughty with the transparent Slayer. Xander, none the wiser, leaves to continue his search for Buffy. After Spike explains his dissatisfaction with her current state, he boots her from the crypt.
At the Espresso Pump, Willow uses her computer to search for answers. Although she is tempted to use magic, she refrains.
After an exchange with Dawn, Buffy listens to a message from Xander, who tells her about the pudding.
Meanwhile, Willow makes her way to the underground lair when the invisible threesome grabs her. Back at Buffy's, the Slayer is about to leave when she is called by the geeks, who tell her to meet them at an arcade.
At the arcade, a kidnapped Willow tells Buffy that Warren is actually trying to kill her rather than save her. Luckily, a fight breaks out and amidst the confusion everyone is made visible again by Willow. Although Buffy recognizes Warren and Jonathan from high school, she doesn't recognize Andrew. Warren explains that they are her arch nemeses. Somehow, the three manage to escape through the back door. Willow informs Buffy that she tracked them down the old fashion way, without resorting to magic.
Summary from Buffy.com