It's Always Sunny in Philadelphia Season 5 Episode 12
The Gang Reignites the Rivalry
When the gang's ten year ban from Flipadelphia, the annual city-wide Philadelphia flip cup tournament, is lifted, the gang goes to challenge their old rivals at dive bar Molly's. However, the gang soons finds out their rivals have now become an upscale restaurant and have grown up and moved on with their lives. However, the gang does not care. Apparently Dee choked under pressure last time and lost them the tournament, so Frank and Dennis head over to Dennis' old fraternity to recruit a new fourth member. When Dennis gets humiliated and tazed by the new frat members, he runs away swearing revenge.
Mac and Charlie agree to exact revenge and head back to the house during a party and fall in love only to be chased out with no new fourth member. Meanwhile, Dennis and Frank head to the owner of Molly's to still try and reiginite the rivalry. Back at Paddy's the gang finds out Sweet Dee has been practicing and is ready to go. They decide instead to get their revenge on the frat and head over and challenge them to a flip cup game. If Paddy's wins, they can party at the frat, if the frat boys win, they can drink at their bar anytime they want.
Dee, once again chokes, but the boys' plan was actually to poison the frat and it works. Back at the bar, the owner of Molly's comes over and is pissed... they never show up to Flipadelphia and he was ready to go after sabotaging. They tell him they're over it and he leaves pissed. Hilarious season finale.