A man tries to get into the library to see Giles but is killed by a female demon. After killing him,she turns into a puddle of green goo. At school, Giles tells Buffy to meet him later at the hospital for a blood delivery, which attracts vampires. Back at the library, a detective is waiting for Giles --the dead man had Giles' address on him. Giles identifies the body as an old friend from London. The body has a tattoo on it, which Giles says he can't identify. Shaken, Giles doesn't meet Buffy at the hospital, and she battles the vampires alone until Angel shows up. When Buffy goes to Giles' house to see what happened, he's disheveled and sends her away. He calls another friend in London and finds out that she's dead, too. As he rolls up his sleeves, we see he has the same tattoo as his friend. Meanwhile, the dead friend, Phillip, comes back to life in the morgue, his eyes flashing, and escapes.

Saturday, Willow, Xander, and Cordelia meet with Jenny Calendar -- Giles' girlfriend -- for a computer class. Buffy shows up to express her concern about Giles' behavior. Cordelia mentions the police visit to the library, which slipped her Cordelia-centered mind. In the library, Buffy finds Ethan, the costume shop owner and Giles old friend who tried to kill her on Halloween. As she calls Giles, Ethan mentions the "Mark of Eyghon." Giles says she's in danger, and the dead Phillip enters. A panicked Giles shows up and, after a scuffle which leaves Jenny unconscious, Phillip turns into the green goo. Some of it gets on Jenny and when she comes to, she seems normal but we see her eyes flash. Research freak Willow discovers the "Mark of Eyghon" in a book: Eyghon possesses the body of a dead or unconscious host. They figure out that he's jumped from Phillip's body to Jenny's. Possessed Jenny, looking and sounding like a demon, tries to seduce Giles. When Buffy comes to the rescue, Jenny jumps out the window. Giles explains to Buffy that he ran with a bad crowd when he was young, and they used Eyghon as a temporary high -- directing him in and out of each others' bodies. But one friend died, then later the girl who killed Phillip, and now it seems the rest of the group will die, too. Buffy goes to the deserted costume shop to try to help Ethan but he knocks her out, ties her up and tattoos her. He pours acid on his own tattoo so that Eyghon will take Buffy instead of him. Jenny enters, a full-tilt demon, and Buffy breaks free. Angel enters suddenly and chokes Jenny: the spirit leaves her body and enters Angel's, since he's really dead (Willow's plan: you go, girl!) It passes through Angel, and Jenny's back to normal, but Ethan escapes.