According to Jim Recap: Old Friends
When Jim gets stopped by a police officer on his way home from work one day, he's surprised at first to see that it's his old friend Danny (Dan Aykroyd) - but even more surprised to see that he's no longer the wild and crazy party animal that he once was, and has become an office of the law. Jim invites Danny to have dinner with Cheryl so that they can catch up on what has happened over the last ten years, and Cheryl is at least as surprised at the change that's taken place. Naturally, Jim takes it as a challenge that he is no longer the wild man, and he and Danny head to the bar.
The next morning, Jim isn't doing so well and Andy takes it upon himself to rub it into Jim that he's got a hangover - Jim doesn't take it so well. Unfortunately, Danny didn't do so well either. He took his ticket book to the bar with him, and when he was handing out tickets to the women, he used his real tickets, which made it back to the station, so he's been suspended. The good news is that means he has more time to spend with Jim, so they head out again that night, and the night after, and the night after. Finally, on the fifth night, Jim has to say he can't make it. Dana and Cheryl want him to go, but he eventually has to tell Danny that he just can't keep up.
Danny, of course, feigns offense, but really he just wants someone to go. Jim ends up meeting him at the bar, and they make up, realizing that they are now past their prime, and they just can't go out to the bar every night. They can, however, get up on stage and perform a song, so they do just that, giving a nice rendition of Cadillac Man.
