"Yang 3 in 2D" is the sixteenth and final episode of season five. 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 employ Yang's knowledge of Yin to solve the case by allowing her to accompany them on the investigation.
A young woman named Allison Cowley comes into the police station claiming to have just escaped after being kidnapped and held by the notorious serial killer, Yin. Shawn, Gus, 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's telling the truth by showing Shawn the picture of himself as a child standing next to Yang. 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.
They all race down to the college to discover there has been a substitute in the class for days. They ask the dean for information on the missing Professor, but everything on his CV turns out to be false. The only real thing on there is a P.O. Box, where they find VHS tapes about Yin and Yang that Mary Lightly had made for Shawn. In them, Mary warns that Yang might be a victim of Yin's and that Yin is angry with Yang for being apprehended and jealous of her newfound infamy. Meanwhile, Henry and Madeleine have tracked down their own copy of the picture and Madeleine remembers it to be taken in front of the neighbor's house. Henry tells Shawn, who rushes to the house with Gus to save Allison. Shawn and Gua find a note in the mailbox instructing them to go inside to save Allison but, when they do so, the door shuts behind them, locking them inside.
Meanwhile, Shawn and Gus have found Allison only to discover she is Yin's new apprentice. She takes them, at gunpoint, to an upstairs room where Yin is sitting with his back turned to them. Allison ties Shawn and Gus to their chairs and then, much to her chagrin,Yin asks her to exit the room. He then stand up and turns around, finally revealing himself as a previously unseen character. He states that even he shudders to think of what Allison will be capable of in the future and then expresses his annoyance with Yang for her lack of passion for murder. He then begins filling a syringe that's he's going to kill them with, promoting Gus to inquire as to what he is putting into the syringe.
Outside, Henry is anxious to get inside the house while they wait for the battering ram. Chief Vick is hesitant to allow Juliet and Yang inside the house by themselves but ultimately agrees, as that is the only way that Yang will agree to lead Juliet through a hidden storm cellar and into the house. Carlton is visibly concerned for his partner but sees that she is determined to her inside the house even if it means going in without backup. Once inside, Yang unexpectedly charges at Juliet and breaks a light bulb as she runs from the basement into the rest of the house. Juliet quickly gets up and follows her.
Back upstairs, Yin describes the concoction that he has filled the syringe with, which he calls "The Last Kiss." He explains that the entire contents of the syringe are intended for Gus, to which Shawn responds by expressing his inability to watch Gus die and asks that Yin kill him first. Gus, in turn, states that he can't stand to watch Shawn die.
On the first floor, Juliet is searching the house and comes across Allison, who looks scared and is holding a shotgun, claiming that she just woke up. Juliet soon figures out that Allison is lying, leading to a violent fight. Just when it appears that Allison has reached the shotgun, Juliet manages to push her into a glass table.
In another part of the house, Shawn and Gus appear to be attempting to stall Yin by asking him to explain his backstory, when Yang shows up and calls him "Daddy." It turns out that Yin had been forcing Yang to kill with him all these years until she finally couldn't take it anymore and let Shawn catch her. She had always longed for the perfect childhood she saw Shawn enjoying as a child, but Yin insists it was far from ideal as the Spencers' marriage was falling apart and Henry ignored Shawn for work. He tells her he was always there for her and pulls her into the fatherly embrace she always wanted. But she knows he'll never change, so she plunges the poisonous syringe into his chest, killing him just as Juliet subdues Allison and the SWAT team arrives. Back in the police station interrogation room, Shawn and Juliet share a kiss as Lassiter walks in to retrieve a notebook he had left there, shocking him immensely.
- As with earlier installments of the Yin and Yang saga, the title employs entertainment imagery, in this case evoking the '3D' screen technology which was just beginning to make a comeback at this time.
- Lassiter's comment about the photo of Shawn and Yang looking Photoshopped to him is also most likely a reference to the fact that at the end of "Mr. Yin Presents," the photo shows Liam James as a Young Shawn, and in this episode, it now shows Skyler Gisondo as Young Shawn, since the actor changed between the seasons.
- Peter Weller, who plays Mr. Yin, also played Stan Liddy in the TV show Dexter. In the show, Stan Liddy becomes obsessed with proving the main character, Dexter, to be a serial killer. Mr. Yin is killed in a similar manner to Stan Liddy, stabbed in the chest.