In structure it is your average dopey sports movie. First time director Barrymore (who also produced) and writer Shauna Cross (who wrote the novel "Derby Girl", which the film is based on) cover this up with lots of action and fun. The sport in question is Roller Derby, which is sort of like Quidditch on wheels. Since most of the audience has no clue, Barrymore kindly includes an explanation of the sport. Roller derby takes place on a circuit track that looks like a vellodrome to me. The two teams each have five players on the track — three blockers, a pivot and a jammer (scorer). Points are scored by the jammers, who earn one point each time they legally pass an opposing blocker or pivot. The sport is very unusual in that it is an all-female contact sport.
In the movie, Bliss (Ellen) must overcome her fears and insecurities in order to, as the tag line says, "become her own hero".