The Yin/Yang trilogy comes to a thrilling close as Shawn and Gus race to save a woman kidnapped by Yin. Only this time, they are forced to use Yang's knowledge of Yin to solve the case by allowing her to accompany them on the investigation.
A young woman named Allison comes into the police station claiming to have just escaped after being kidnapped and held by the notorious serial killer, Yin. Shawn and the police are loath to believe her since crackpots have been coming into the station for months making similar claims. Yet Allison proves she telling the truth by showing Shawn the picture of himself as a child standing next to Yang that we saw at the end of last season. With the cops on board, Allison takes everyone back to where she claims she was held only to find the house occupied by an innocent family. Shawn still believes her though, and puts her up in Henry's house while they investigate further. The crime lab determines the picture to be real but the background is fake. This could have been taken anywhere.
Back at Henry's house while Allison is asleep upstairs in Shawn's childhood bedroom, Henry hears a noise outside and goes to investigate. When he finds nothing, he rushes upstairs to check on Allison but finds only a Yin Yang symbol painted onto the bed instead. With Allison gone again and the clock ticking, Shawn turns to Yang for assistance on finding her partner, Yin. She agrees to help, but says she needs out of the psychiatric prison to do it. They all arrive back at Henry's, who has been trying to figure out where the picture was taken and even called Madeleine for help. Yang is thrilled to be in Shawn's old bedroom and tells him not to look for something Yin left behind this time, but to look for something he took. Shawn discovers a particular 45 record of his is gone which leads them all to the Junior High teacher he had a crush on. When this lead turns out to be a dead end, Yang warns his thinking is too remedial. Shawn then realizes the teacher they are looking for is Allison's.