Season 3: Episode 29: The Thin White Line
Brian feels like he's stuck in a rut, and so he takes his therapist's advice and tries to help others for a change. He ends up a very successful drug-sniffing dog for the Quahog police department, but along the way he gets addicted to cocaine. Peter is disappointed when the Griffin family has to cancel their planned vacation in the Bahamas to help support Brian who is in rehab, but decides to tag along with his dog when he sees the beautiful clinic. A therapist at the clinic tells Brian that Peter is a bad influence on him. Although Brian defends his friend and leaves rehab in protest, the wise dog realizes he does need some time away from the Griffin family and leaves Quahog for parts unknown.