Chronologically, "Hit-Girl" takes place between the first and the second mini-series of the parent title, and serves to further delineate the motivation between the key players, most notably Mindy, the 12 year old assassin. As the story starts, she has moved in with her mother and the police officer foster father, trying to balance the life of an ordinary school girl with her nighttime activities as a vigilante serial killer.
It goes without saying that the book is completely over the top. Hit-Girl's bizarre attempts at fitting in the school's social structure are played as a parody, with the child assassin showing her classmates the same ruthlessness that she dishes out in her superhero escapades. Unfortunately, Romita junior's depictions of children leaves a lot to be desired, as most of Mindy's peers are rendered in a way that bears only cursory resemblance to actual human anatomy. The artist has spent his whole career in the superhero industry, and naturally feels much more assured when tackling the scenes dealing with out of school activity, which thankfully make up most of the book.
Besides the main narrative detailing Hit-Girl's troubles with the secret identity, Millar weaves several subplots, with the girl's relationship with Kick Ass being perhaps the most interesting. Using the knowledge thought to her by her late father, the hyper competent protagonist spends the first half of the book drugging her parents and going out to train the five years older Dave, who also doubles as a sympathetic school friend. This kind of a reversal from the traditional superhero/sidekick dynamic is relatively fresh for the medium, and certainly in keeping with the book's general tone, but once Kick Ass suffers a minor injury, the character's arc more or less ends.
Millar uses the remaining space to spotlight more of the villainous Red Mist's origin, which is entertaining, despite being largely divorced from the main plot. The fact that Dave's nemesis doesn't get to play the antagonist here presents a major problem, as the criminals opposing Mindy never outgrow their roles as one-dimensional foils. Red Mist's scenes ultimately serve to further explain his motivation and set up his actions in the second series, while also reminding the reader of his role in "Kick Ass 3".
The gangsters Hit-Girl dispatches left and right end up threatening her family, by way of blaming her foster father for refusing to actively side with the Genovese family and their associates. The plot contrivance, coupled with smart scripting leads to a conclusion that epitomizes Mindy's abilities, but also leaves her largely unchanged for the beginning of the second "Kick Ass" miniseries, which it precedes chronologically.
This is another, and perhaps the most profound of the book's weaknesses. Despite seeding the debuts of some of the minor superheroes and providing the details regarding Mindy's secret identity and her family dynamic, the book is largely static. "Hit-Girl" serves to provide fans with more of the breakout character, but is largely superfluous to the main plot, while being completely anti-climatic. Having read "Kick Ass 2", the reader is aware of the ultimate outcome of the characters and their struggle, robbing the book of the shock factor that has provided so much of its appeal.
Millar and Romita junior could have utilized the spin-off to subversively introduce a major element, such as a romantic subplot between two teens (which would have been shocking considering the age difference), but they seem satisfied with providing another look at the ruthless world of the Kick Ass. The reader is offered a slight plot, filtered through the eyes of a side character, but at least in that respect the book is a success.
Mindy will never be as shocking as she was when she debuted in the original "Kick Ass" mini-series (as well as the movie adaptation), but she is still reasonably entertaining and, more importantly, works as a lead character in her own book. The creators' mandate seems to have been to present a well paced book with high production values, in order to keep the attention on the property while the second movie is being produced. Despite sacrificing some of the original's notoriety, "Hit-Girl" likewise works as a lighthearted tie-in, published in anticipation of the creative team's supposed final work on the subject, the "Kick Ass 3" mini-series.