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 their old Sunday school teacher, Father Westley. Lassiter and Juliet are already on the scene, speaking with Westley and headmaster Father Bard. Westley and Father Bard explain that Agatha was a model student until three weeks ago when she suddenly started acting irrationally. Lassiter thinks it's murder but Shawn think the death is a case of suicide. Father Westley, however, has an alternate theory, he believes Agatha was possessed. Father Westley's idea is disregarded by all but Gus. Father Bard, who also dismissed Father Westley's idea, explains to the four detectives that Westley used to perform exorcisms in the past. Father Bard also informs the group that exorcisms are a practice that the Church now looks at with skepticism. Westley though, is insistent and Gus agrees to take the case, even while Shawn objects.
Westley tells Shawn and Gus more about Agatha's strange behavior. He had wanted to perform an exorcism but couldn't get approval from Bard, as the practice was frown upon. Shawn, still skeptical from his childhood experience with Father Westley, thinks maybe Agatha was just a disturbed girl. Father Westley gravely counters, saying he's dealt with disturbed teenagers before and that Agatha was different.
Back at the Psych office, Gus is doing research on Agatha and has logged onto her blog. 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 a a few days before her death. To Shawn, this evidence proves that an alternate theory to demonic possession is possible. This being the case, Shawn and Gus go to talk to Cameron.
Cameron says Agatha broke up with him even with his passionate plea to work the relationship out. He also has a solid alibi, as seen by Shawn, for the time of Agatha's 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 They then pose as grief counselors and head to Lucy's house, where her mother informs them Lucy has become withdrawn and is only a shell of her former self. Shawn and Gus head up to Lucy's room to talk to her, and when they open the door to her room, they 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, Shawn however, is adamantly sure that there is another explanation. Shawn then leaves Gus to get advice from Henry on how to deal with an irrational partner. Meanwhile, Gus calls in Father Westley to help with Lucy. Shawn returns to Lucy's as Westley and Gus are preparing to enter the room to preform the exorcism. If it works, the case will be closed Gus says when Shawn asks if they can go back to psychically doing things if the exorcism doesn't work. Shawn is skeptical, even during the exorcism as Lucy starts spouting out childhood information about Shawn. Father Bard then appears, Shawn and Gus are ordered out of the room. After what seems like a few minutes Westley and Bard finish the exorcism, emerging later to claim it worked and that Lucy is now cleansed of her demonic possession.
Shawn, still unconvinced and skeptical, goes to the police station for more information. Juliet tells him the standard toxicology report came back clean, and that they could administer a more specific test if Shawn had a particular medicine or drug that he wanted to check for. Shawn looks through Gus' index of medications and gets an idea. Later, after a second toxicology report, he tells Gus that the cops found L-Dopa in Lucy's system. L-Dopa is a Parkinson's medication that, when taken by a person without Parkinson's causes extremely emotionally erratic behavior.
The girls were not possessed, Shawn deduces that the girls were secretly being slipped the medication. Gus asks how Lucy knew about Shawn's childhood and how to speak German. Shawn counters that Lucy viewed 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 an exorcist, fed the girls his medicine to bring on the symptoms, but that things got out of hand when Agatha killed herself. Lassiter and Juliet arrest Westley. There in jail, Gus visits Westley, whom proclaims his innocence. He knows who the culprit is but he can't tell Gus because he heard it in confession. Father Westley then tells Gus that a certain proverb has been of great comfort to him lately.
Gus goes looking for Shawn, and finds him down at Henry's, telling him about Westley's innocence and his reference to the proverb. Shawn and Gus find Henry's Bible, and see that the proverb mentions the unfaithful and evil desires. Shawn then puts it all together and, with Gus, Lassiter and Juliet, go to see Cameron, and find Lucy in Cameron's room. Shawn begins with his theory by saying that 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. Shawn then goes on to say that when Lucy overheard Westley proclaim Agatha was possessed, she realized she had the perfect thing to throw the cops off the trail. Upon hearing this, Lucy breaks down, claiming she never meant to hurt Agatha, which leads to the cops cuffing her and dragging her away.
- Whilst the 'Upstairs Bedroom' rider refers purely to exorcism (and the film), the first part of the title is a quotation of uncertain origin, a variant on the concept that 'God is in the detail', and designed to express the need for precision and care, in matters beyond human comprehension particularly.
- The 1973 horror film "The Exorcist" contributes many elements and references to the show in this episode.
- Possessed Lucy speaks German: "Jeden Tag triffst Du eine Entscheidung, die Dein Leben verändern wird" ("Every day you make a decision that will change your life").