A man hurries nervously across the Sunnydale High campus at night, looking for Rupert Giles. Hearing a noise behind him, he turns to see the decaying form of a woman he knows, Dierdre, her eyes gleaming. He runs to the library door and pounds on it, screaming for help. Buffy and Giles are within, but hear nothing because Buffy is aerobicizing to loud music. So they hear nothing as Dierdre steps close, closes her hands around the man's neck, and kills him. As soon as his body hits the ground, she also collapses, dissolving into a puddle of blue-green goo.
The next day, after a night disturbed by nightmares, Giles reminds Buffy of their plans for that night, to monitor a monthly delivery of blood to the hospital for possible vampire activity. When Ms. Calendar arrives with a reminder of that Saturday's computer tutorial for Willow (her helper) and Xander (her victim), romantic sparks fly between her and Giles. But his good mood is wiped out by the police officers waiting for him in the library with news of the man found dead outside the library. He goes with them to the morgue and identifies the man as Philip Henry, an old friend he hasn't spoken to in twenty years. Philip has a strange tattoo on his arm which seems to disturb Giles, but he tells the police he doesn't know what it is. That night, Giles never shows at the hospital, but with Angel's help Buffy stops the vampires from ruining the blood delivery. Concerned, she goes to Giles' house. He answers the door looking tired and awful, curtly dismissing her. A phone call to England reveals that another old friend, Dierdre Page, has died recently. Rolling up his sleeve, Giles looks at the tattoo there (the same one Philip had), then at his own haggard face in the mirror, saying, "So... you're back." Meanwhile, at the morgue, Philip's corpse pulls the sheet away from its face, and his eyes open, gleaming.
The Saturday computer tutorial is interrupted before it begins by the arrival of Buffy, who reports Giles' odd behavior. When Cordelia mentions Giles' meeting with the police, Buffy goes off to the library to call him. When she gets there, she hears something and goes into the stacks to investigate, where she's attacked by the intruder — Ethan Rayne, the guy from the costume shop. She subdues him and calls Giles, but the call is cut short when Philip's corpse walks through the library window. A melee ensues, during which a strategically placed knee by Cordelia prevents Ethan from escaping, and Buffy gets Philip locked in the book return cage. Giles arrives and confronts Ethan, but Philip escapes, knocking Ms. Calendar unconscious before being rendered equally unconscious by Buffy. Philip dissolves into a puddle of blue-green goo, which seeps over Ms. Calendar's hand before she regains consciousness. She hugs Giles for comfort, but her eyes gleam.
When Buffy demands an explanation for the strange events, Giles clams up and orders her to stay out of it. Of course, that doesn't stop her — she sets Willow, Xander and Cordelia to work on researching the mysterious "mark of Eyghon" Ethan told her about. Willow finds the answer: Eyghon is a demon that possesses an unconscious or dead host to live, but if the possession is not undone, it becomes permanent and Eyghon is "born from within the host." They realize the demon must have possessed Ms. Calendar, and Buffy goes off to try to stop it. At Giles' place, Ms. Calendar becomes extraordinarily aggressive in her come-ons to Giles, but when he pushes her away, refusing to take advantage of her in her weakened condition, she takes on a demonic visage and voice and attacks him. Buffy bursts in and drives the possessed computer teacher away, and Giles finally tells her the story: how, rebelling in his youth against the pressures of Oxford and his destiny as a Watcher, he fell in with a bad crowd and summoned Eyghon for the rush of it. But they lost control of the demon, and now Eyghon has returned to kill them all. Only he and Ethan are left. Buffy goes to the costume shop to try to protect Ethan, but he knocks her unconscious.
The frustration of trying to come up with a solution almost has Xander and Cordelia at each other's throats, but Willow stops them and then has a brilliant idea. Meanwhile, Ethan has tattooed Buffy with the mark of Eyghon and used sulfuric acid to remove his own. When the demonic Ms. Calendar shows up at the costume shop, she pushes Ethan aside to get to Buffy, but Buffy breaks free and fights Eyghon. Then Giles shows up, demanding that Eyghon take him instead. At last, the cavalry arrives — Willow and friends have brought Angel, who attacks Ms. Calendar. In danger, the demon leaps into the nearest dead host: Angel, whose demon fights and destroys Eyghon. Ethan has disappeared again, but "It's over," as Cordelia observes. It does indeed appear to be over: Ms. Calendar is distant with Giles, not even wanting to be touched. She has suffered too much. Buffy tells Giles how surprised she is to learn that grownups can be people too.
