After Agatha, a student at a Catholic college, jumps off the roof of a building, Gus drags Shawn to the school to investigate as a favor to an old teacher, Father Westley. Lassiter and Juliet are already there speaking with Westley and headmaster Father Bard, who explain that Agatha was a model student until three weeks ago when she suddenly started acting irrationally. Lassiter thinks it's murder but Shawn is leaning towards suicide. Father Westley has an alternate theory. He believes she was possessed. Father Westley is roundly dismissed, even by Father Bard, who explains that Westley used to perform exorcisms in the past, something the Church now looks on with skepticism. But Westley is insistent and Gus agrees to take the case, over Shawn's objections.
Westley tells them more about Agatha's strange behavior. He wanted to perform an exorcism but couldn't get approval from Bard. Shawn thinks maybe Agatha was just disturbed but Westley says he's dealt with disturbed teenagers before and this was different. Back at the Psych office, Gus is doing research on Agatha and has logged onto her internet page. He charts her mental demise by the content of her status updates. Shawn scrolls through her site and discovers that Agatha had a boyfriend named Cameron, but they broke up shortly before her death. To Shawn, this proves an alternate theory to demonic possession and they go to talk to Cameron. He says Agatha broke up with him even though he wanted to work it out. He also has a solid alibi for the time of her death. Shawn and Gus discover that Agatha's best friend Lucy saw her jump, and cradled Agatha in her arms while she died. They then pose as grief counselors and head to Lucy's house, where her mother informs them Lucy has become withdrawn and is only a shell of her former self. They head up to talk to Lucy, but find her spider walking around her room, acting violently and speaking in foreign tongues - common signs of possession. Gus thinks the demon probably jumped into Lucy as Agatha died but Shawn is sure there's another explanation. Shawn leaves to get advice from Henry on how to deal with an irrational partner and Gus calls in Father Westley for help. Shawn returns to Lucy's as Westley and Gus are about to start the exorcism. If it works, the case will be closed. Shawn is still skeptical, even as Lucy starts spouting out childhood information about Shawn. Father Bard then appears and Shawn and Gus leave as they finish the exorcism, emerging later to claim it worked and Lucy is now clean.
Shawn is still unconvinced and goes to the police station for more information. Juliet tells him the usual toxicology report came back clean but they could administer a more specific test if Shawn had something particular in mind. Shawn looks through Gus' index of medications and gets an idea. Later, after a new toxicology report, he tells Gus that the cops found L-Dopa in Agatha's system. It is a Parkinson's medication that, when taken by a person without Parkinson's causes extreme emotional erratic behavior. The girls were not possessed. They were secretly being slipped the medication. Gus asks how Lucy knew about Shawn's childhood and how to speak German. Shawn counters that Lucy logged on to his internet page and that her German was a quote from the movie Run, Lola, Run. He then tells Gus that Westley has Parkinson's and has been on L-Dopa. It looks like Westley, wanting to return to his glory days as exorcist, fed the girls pills to bring on the symptoms, but things got out of hand and Agatha killed herself. Lassiter and Juliet arrest Westley and Gus visits him in jail, where Westley proclaims his innocence. He knows who the culprit is but he can't tell Gus because he heard it in confession. He then tells Gus that a certain proverb has been of great comfort to him lately.
Gus tracks Shawn down at Henry's, telling him about Westley's innocence and his reference to the proverb, which mentions the unfaithful and evil desires. Shawn then puts it all together and, with Gus, Lassiter and Juliet, goes to see Cameron and finds Lucy in his room. Shawn says Lucy wanted Cameron for herself. She drugged Agatha to make her crazy so Cameron would dump her. Unfortunately, the medication made Agatha so unstable, she jumped to her death. When she overheard Westley proclaim Agatha was possessed, Lucy realized she had the perfect thing to throw the cops off the trail. Lucy then breaks down, claiming she never meant to hurt Agatha, as the cops cuff her and lead her away.