Mac's Mom Burns Her House Down
It's Always Sunny in Philadelphia Season 6 Episode 6 (10/21/10)
When Mac's mom burns her house down, the gang suggest she moves in with Charlie's mother. Mrs. Kelly is reluctant, but moderator Dennis arranges the deal. Mrs. Mac, minus her sick old dog, Pippin, moves in and the two initially don't hit it off until Mrs. Mac takes control of the situation. In the end the two seem to be doing fine, so the gang goes to recover Pippin from Sweet Dee...
Speaking of Sweet Dee, she's a little sick and trying to get better in time for her backstage adventure with Josh Groban. Frank, worried no one will take care of him when he's older, goes to try and help nurse Dee back to health. Dennis, pops in, helps his father by drugging Sweet Dee and moving her to an uncomfortable location... Frank's house. They end up drugging her until she misses the concert. Plus Pippin gets into the drugs and dies.
Dennis and Frank throw Pippin the trash, where Charlie and Mac find him... alive.