Plot (Spoiler)

When Brian is hurt, the Griffins think he may be getting too old. Peter buys a younger dog, whom he calls “New Brian”. New Brian’s upbeat attitude makes Brian feel unloved (especially when the Griffins begin ignoring him), and he decides to leave home. New Brian’s relentless cheerfulness begins annoying Stewie, who asks Brian to return. Brian refuses, saying he will not come back if New Brian is there. Stewie tells New Brian he does not like him, and New Brian retaliates by revealing that he violated Rupert (Stewie’s teddy bear). Angered, Stewie kills New Brian and forges a suicide note; the Griffins then take Brian back.