I’m not going to bother writing a summary, since so many others have already done a mighty fine job of doing so (subculture, randomc, fukazenna!, Tenka Seiha et al.), so I’m just focusing on my general thoughts.
But in case you’re really lazy, here’s what happened. Girl enters high school. Girl needs to join a club. Club needs a girl or it gets shut down. Girl and club meet. Voila! Now they have a band.