The Man with Two Brians
Eight-year-old Brian (who is 56 in dog years) seems to have lost a step or two, so Peter gets a new talking dog, New Brian, to "take some of the load off" the old one, who doesn't like the idea at all.
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After watching Jackass, Peter and his friends decide to film their own highly dangerous stunts just like the show. In one stunt, Peter attempts to jump a lake in his Greatest American Hero costume but instead crashes into a tree, causing him to fall into a lake and rendering him incapacitated. Brian swims out to save him from drowning, but strains his back during the rescue, leaving him stranded in the water along with Peter. Joe then saves them both.
Brians2
Back at home, Lois chastises Peter for causing Brian's condition, explaining that Brian could have drowned out there as well, since he is getting old, has been smoking and drinking all this time, and should not be trying to save Peter's life under those conditions. Peter begins to grow upset about Brian's age, so he gets another dog named New Brian. The new dog's positive attitude and desirable personality make Brian feel like an outcast and ultimately cause him to move out. Brian ends up staying at both Cleveland's and later Quagmire's house.
Meanwhile, New Brian's constant cheerfulness starts to aggravate Stewie. Stewie pleads for Brian to return, but Brian says that he can't as long as New Brian is there. So Stewie tells New Brian that the Griffins want him to leave, claiming that they do not like several things about him, including how he humps the leg of one of their chairs. New Brian reveals that he also humped Rupert, Stewie's favorite teddy bear, and boasts about it. Stewie at first appears unfazed, but then murders New Brian and forges a suicide note. The Griffins happily accept Brian back. Later, Stewie, clearly traumatized by what New Brian did to Rupert, frantically washes the bear in the shower and whispers "I don't blame you" to him.