In it, Brendon is in high school, already out of his parent's house, and living with Shane (who is his ex-boyfriend and current best friend). He plays a ton of instruments, makes money by giving music lessons, and has a recording studio in the basement of their house. He produced a demo for his neighbor (who I think ended up being Ian/The Cab?). (At one point someone asks if there's anything Brendon can't play, and Shane says he can't play the tuba, but Brendon says he's awesome at the tuba but Shane won't let him play it in the house).
He auditioned for Panic! (or whatever they were called in this fic) and didn't get in - Ryan turned him down because he was into some girl and wanted her to do it instead, I think. But Spencer ended up finding out about the instruments and producer stuff and said something to Brendon like "We thought you were just a kid like us." And then he went over to Brendon's place to jam and record some stuff. They hooked up, but Spencer was gone in the morning and never called Brendon.
Brendon later went to a party and Ryan and Spencer were there. Ryan had missed some details about what happened that night, so he accused Brendon of having stolen Spencer's beats for his own music, so Brendon punched him.
They ended up talking it out and Brendon and Spencer made up, but I don't think they started dating right away, and Brendon took Spencer's drumming out of his demo. They agreed to stay out of each other's music.
Spencer/Ryan's band ended up getting signed by Pete (with a demo that Brendon produced) and go to LA to record. They all felt uncomfortable with the fact that Brendon wasn't there with them. Pete finds out about Brendon and Spencer gives him his ipod that has Brendon's music on it - (Brendon doesn't want people hearing his music yet, so Spencer pretends to have "lost" his ipod and that Pete "found" it). Pete's like "hot damn!" and Brendon gets signed as a solo artist. Brendon and Spencer were dating by this point for sure.
I think there was another part to the fic where Brendon was on tour a few years later. His backup musicians were a band of their own called something funny - Taco Ninjas??? it was weird like that lol. I think by then, he and Spencer were broken up or on a break at least. Spencer's band breaks up and he felt super betrayed by Ryan (and Jon?) and goes to Brendon's tour and they get back together.