Dr. Steven Reidman is an expert on the Thornburg Virus. When the virus is released in a coffee shop, the SBPD along with Shawn and Gus visit Dr. Reidman to see if he thinks it'll spread, but he says only two are showing early signs of Thornburg, so the crisis is most likely averted. Reidman then begins administering the antidote to the patients.
Later, the man who stole the Thornburg and let it out in the coffee shop escapes, and they find out that they didn't send one vial of Thornburg, they sent three. Reidman states that he's very interested in Shawn's "psychic abilities", and Shawn promises he'll give him a sample of his DNA after the case is over. The man infecting everyone, Dr. Tony Mallon, dies of Thornburg, but they find his stash of anti-viral, which they will use to cure Juliet. Shawn has an epiphany that he needs to tell Juliet how he feels now, before it's too late, but when Dr. Reidman lets Gus know that she doesn't actually have Thornburg, Gus interrupts Shawn, and Shawn chokes and leaves.
After the case is over, Reidman comes to the Psych office and takes a sample of Shawn's DNA for examination, and measure's Gus's head.