Rachel Watts lives two doors down from the brilliant, troubled and eccentric James Mycroft, the most intelligent boy in school. What he has in abundance in the IQ department, he lacks in social smarts, and as his closest friend, it seems to be Rachel's job to try to keep him out of scrapes with other students at school (who he will frequently inadvertently or purposefully insult) or from getting suspended for breaking school rules. She also tends to bring him food, as he has a tendency to forget to eat when left to his own devices. While he lives with his aunt Angela, they barely speak and seem to have a mostly antagonistic relationship.
Mycroft, who likes to compare himself to his fictional namesake, Sherlock Holmes' older and allegedly smarter brother, has a passion for forensic pathology and frequents message boards online and writes articles, even occasionally consulting on cases, under the name Diogenes. Watts helps him proof-read and edit the articles. When the teenagers find "Homeless Dave", a man they regularly visit and bring food near Melbourne zoo, murdered not far from his local sleeping spot, Mycroft uses everything he knows of forensic experience to document the scene before the police arrive. He later persuades Watts to be his side-kick in earnest, determined to investigate the death that the police undoubtedly won't care much about, as the victim was a homeless nobody.
Rachel used to live with her parents and older brother Mike on a sheep farm in the country. Financial difficulties forced them to sell the farm and relocate to Melbourne, where Rachel's parents and brother work hard to make ends meet. Rachel wants to study agriculture and move back to the countryside, she feels uprooted and unsettled in her new urban surroundings. With her academic achievements, her family want her to go to college and get a proper degree, though. Previously home schooled and used to a solitary life, the bustling corridors of her new high school and the constant stress and noise of the city is making Rachel miserable. Having made good friends in Mycroft and the fierce Mai Ng, as well as Mai's boyfriend Gus, makes her existence more bearable, but she's still not happy in Melbourne.
James (who always goes by Mycroft) is English and lost both his parents in a horrific car crash that left him scarred both physically and emotionally. His aunt Angela is his legal guardian, but they may as well be strangers just living in the same house for all the time they spend together. Mycroft is constantly skirting the edge of having social services investigate his living situation, which while not idyllic, is at least better than a foster home. Mycroft loves mysteries and is a keen observer of everything around him. While fiercely intelligent, he's also low on social graces and frequently pisses off his class mates or gets into trouble with the school management. He's obsessed with finding out the cause of his parents' accident, and frequently emotionally unstable, with Rachel, sometimes aided by Mai and Gus, doing her best to keep him from getting beaten up or expelled.
Rightly surmising that the police are unlikely to expend too many resources on trying to solve the murder of a homeless man, especially one who appears to have been killed for sport, Mycroft insists that he and Watts need to do their best to figure out who's behind the murder. Rachel initially refuses, but is unable to resist the lure of the mystery or Mycroft's charismatic persuasion for long, and soon the two teens are using everything they know of forensic pathology to identify the killer of their homeless friend. Hunting killers is a dangerous hobby, though, and before long Mycroft and Watts are courting danger and find themselves in near-death situations of their own.
I'm sure it comes as no surprise to anyone that there is also a romantic sub-plot in the book, which is very well done, for all that I'm not sure Rachel should involve herself with someone who is clearly not the most emotionally or mentally stable person (I speak from experience here, Watts). Both Rachel and Mycroft are very engaging protagonists and there is a good supporting cast in the book - Rachel's parents and Mike, her brother, as well as Mai, their loyal friend who more than once uses the legal knowledge she's picked up in school, and Gus, her sweet and funny boyfriend.
I really liked this book, and am going to do my best to track down an e-book store which will legally sell me the sequels online, so I don't have to wait for the US release of the next two books before I can read them.