Mind-altering Murder — William Rabkin

When Shawn's partner Gus decides he doesn't want to be a detective anymore and gets a real job, Shawn wonders: Is this the end of Psych? Or is it the end of Gus?

After a fellow executive at Benson Pharmaceuticals turns up dead, Gus realizes he needs Shawn more than ever to solve the murder before he's forced to take an early-and permanent-retirement.