Ep 8. Season 4, Episode 16: "Mr. Yin Presents..."

James' Pick: When Shawn arrives late to meet Gus at a screening of Psycho, he has to squeeze down the row past other theatergoers to take his seat. On their way out of the theater, they are surprised to meet Mary Lightly (Jimmi Simpson), who helped them track serial killer Mr. Yang. Mary takes them to a diner, where they discuss Yang's new book that has just been released and Mary points out that, according to the book, Yang could not have been working alone. Shawn and Gus don't believe it, and leave the diner - and the Yang case - behind. Later they are called to a crime scene to find the waitress from the diner murdered and her body arranged into a Yin/Yang symbol.