Same Time, Same Place
Buffy, Xander and Dawn wait at the airport for Willow to return from England. Buffy is a little nervous about what to say, but Xander isn't concerned, even after Buffy mentions that Willow left her training early. They continue to wait until everyone has left the plane, but Willow doesn't show up.
Seems that Willow did leave the plane, hoping for a reunion with her pals, but she walked right by them--they were invisible to her. And vice versa.
That night, a boy tags a wall with spray paint, until his work is cut short by a long-nailed, shadowy figure that jumps out at him with an unearthly scream. Apparently, he should have found a more constructive use for his artistic talent.
Willow goes home to an empty house. She reacquaints herself with the room she shared with Tara, briefly reliving the moment she lost her. There's no one around to comfort her, so she lies on the couch, despondent. During those same moments, Buffy, Xander and Dawn get home to an equally empty house. They commune on the couch, not realizing they're sharing space with Willow, who's there, but not.
The next day, a solitary Willow heads over to the dilapidated Magic Box. Anya is still cleaning the debris out of the store, and she's not exactly thrilled to see the woman who destroyed it in the first place. Ultimately sympathetic, Anya sends Will in the direction of the new high school, since Buffy's got "some kind of job helping junior deviants." Willow doesn't find them on school grounds, but she does find something particularly gross: a dead body.
Of course, Xander and Buffy are there, checking out the very same body of the dead spray-painting boy. One detail spooks them: the body is flayed. Their first thought: Willow's back.
At the school, Willow finds Spike talking nonsense in the basement (surprise!). He's no help to her after she explains about the murdered boy, but it turns out he's actually having two conversations: one with Willow, and one with Buffy and Xander. Buffy incorrectly infers from Spike's confused chatter that Willow is involved in the boy's death.
Willow goes back to Anya, who reluctantly agrees to help with a locator spell to find demons in the area. After the spell indicates a powerful demon in the woods, Anya confides that wreaking vengeance all over the place isn't as fun as it once was. Willow relates completely, and the two come to an understanding.
At the Summers house, the gang researches demons that flay skin. Though Buffy still suspects Willow, Dawn finds a demon called Gnarl that sounds about right: it flays, and eats, the skin of its victims. Charming. They enlist Spike to track the demon, since it seems the baddie will have left a trail of blood to its abode. Spike leads them to a cave not realizing Willow is there as well. Willow crawls in the cave at the same time the gang does, and when Gnarl takes a swipe at Dawn, the Scoobies bolt and seal the cave, trapping Willow inside. Gnarl taunts the witch for being left behind by her friends, then lashes out at her, paralyzing her with his poison.
Back at home, Dawn is paralyzed from the poison as well, and Buffy learns that the paralysis lasts until Gnarl is killed. When Anya arrives to stay with Dawn, she reveals that Willow is probably still at the cave, fighting the demon on her own. Suddenly Anya's demon expertise is in demand, and her friends drag her off to the cave to help Willow.
Gnarl is enjoying carefully peeling strips of skin from Willow's stomach when the gang bursts in. They still can't see Willow, but Anya can. At the vengeance demon's instruction, Buffy gouges out the eyes of Gnarl and kills him, releasing both Willow and Dawn from their paralysis. Soon, an injured Willow reappears before their eyes, incredibly grateful that her friends haven't left her alone.
In the morning, Buffy interrupts Willow as she's meditating, using magic to heal herself. Willow reveals that she inadvertently made herself invisible by thinking she wasn't ready to see her friends. Buffy regretfully admits she suspected Willow was the killer, but her friend understands. Weakly, Willow continues to meditate, but Buffy reaches out her hands to help, and they begin to heal her injuries together.
Summary from Buffy.com