Season 15 of It’s Always Sunny In Philadelphia finally showcases the Gang’s origin story. Throughout the series, the Gang’s general history and the things they were doing before the show started had been referenced occasionally, and sometimes even given brief flashbacks in an episode. Season 15’s “The Gang Buys A Roller Rink”, however, is the first time the show has dedicated a full episode to the Gang’s past, and it does so in a very big way.
“The Gang Buys A Roller Rink” opens with Charlie telling the Gang that the roller rink they used to hang out and work at when they were younger is going out of business. Despite the title of the It's Always Sunny In Philadelphia episode, the idea of buying the roller rink is only briefly floated by, with the Gang saying that they already tried that once back in 1998, the last time they were at the roller rink. The episode then proceeds into a flashback showing everything the Gang was doing that night and what led up to their failed attempt at buying the roller rink in the past.
While the flashback was framed as the Gang’s past attempt at buying the roller rink, “The Gang Buys A Roller Rink” was actually an origin story for how the Gang became who they are. Danny DeVito's Frank was the only one whose introduction into the Gang was shown, and the development of his personality was subsequently shown from there. Everyone else, however, has the moldings of their current status quo fully showcased in one of the latest episodes of It’s Always Sunny In Philadelphia.
For starters, the episode has Dennis becoming disillusioned with Frank after Frank, due to some miscommunication, had him watch him have sex with a prostitute, thus providing a specific reason for why they were estranged at the beginning of the show, and Mac’s career as a drug dealer previously referenced in season 7’s “The High School Reunion” evidently came to an end when Charlie, played by Charlie Day, made him realize that he was bad at it to the point that it was putting him in danger. Charlie, meanwhile, was shown getting a taste for huffing fumes when he enjoyed the smell of the aerosol spray, so it’s clear that that was the beginning of his drug problems that destroyed what little intelligence and competency he had in the past, as shown in the episode. The episode also shows the beginning of the Gang taking advantage of Charlie when Dennis and Mac try to convince Charlie to just be equal partners in an investment, despite having far more money than either of them, combined; when Charlie brings it up, they just change the subject to sandwiches, likely a reference to how Charlie would become so bad at business that he would sell his shares of the bar for half a sandwich.
Kaitlin Olson's Dee had the biggest contrast to the present day, though. Her old nickname of “Sweet Dee” often appeared to be used ironically since she was just as crass as the rest of the Gang and was also constantly made fun of by them, but this episode shows that Dee really was nice and innocent to the point that the Gang went out of their way to not make fun of her. All of that changed, however, when Dee got a concussion that made her immediately start acting how she normally does; it’s likely that the Gang also stopped treating her well because her shift in her personality made them feel she was now just as bad as the Gang.
Finally, the episode ends by showing how the Gang came to own Paddy’s Pub in the first place. Dennis, Mac, and Charlie tried to buy the roller rink after Charlie heard his boss had to sell it, but it turned out that he was actually talking about Paddy’s Pub, so they decided to buy that, instead. Dee would undoubtedly join them at some point, Frank following when Danny DeVito joined the cast in season 2, and with that, the main cast of It’s Always Sunny In Philadelphia would be complete.