Fletcher Moon is a natural born investigator. Knowing this, April, a girl from his school comes to him for help in finding a lock of hair that she believes to have been stolen. Fletcher agrees to help her and starts off by investigating all suspects which eventually gets him threatened by a thirteen-year-old named Red Sharkey. Fletcher is a graduate from the Bernsteins Academy, so he is a "certified" detective. While Fletcher is investigating the stolen lock of hair he has a hurling stick thrown at him and is seriously injured. Fletcher holds Red as a prime suspect as the hurling stick had 'Red' embossed on it and comes to the conclusion that it was Red who attacked him. Fletcher then goes to visit April’s cousin May to photograph this newfound evidence but ends up at the wrong house where he catches sight of somebody setting fire to May’s lucky dancing costume. He passes out in the yard due to the anesthetic from the hospital and later awakens to find a torch in his hand and all evidence for the arson pointing to him. Following an interrogation by the police, Fletcher is rescued by Red Sharkey. Red claims he was framed for the hurl assault. Half-Moon and Red team up to solve the chain of mysteries within 24 hours. Following a suspicious story to April's house, the boy detectives discover the truth behind Les Jeunes Etudiantes, the girls club, their true goal being to get rid of all the boys ruining their education. Three recent expulsions can be attributed to these girls, and the boys feel they need to be stopped. Unfortunately, they are found out, and the girls manage to imprison the boys in the cellar. They are, however, saved by May, April's cousin. Fletcher and Red previously heard the girls practicing the lines that they planned to use to accuse Red's brother, Herod of assault. Red is able to send a text message to the present police officer informing him of the lines they would use. The police officer puts two and two together and exposes the girls as liars, and Herod Sharkey is found innocent, saving him from expulsion. April, being caught, attempts to escape in her father's car but promptly crashes into the police cruiser. However, Fletcher is informed that this still does not solve the mysteries that included his assault so he meets a secret informant, and in exchange for the password of the police account he hacked earlier in the story, receives the information that he desires. He finds that the link between the crimes ended up being the upcoming talent show at his school and it turns out that the victims each had a part in the show in some way. After trying to protect May from becoming involved with the criminal Fletcher at last finds the answer he had been looking for all along. All the victims of these crimes had been ranked higher than May in the talent show. So, Red suggests that May was behind all this. However, on stage, Fletcher proves that it was May's father, Gregor Devereux who was the culprit. In the epilogue, Red and Fletcher decide to form a detective group. Red suggests "Moon Investigations" while Fletcher replies by saying "You're half right", referencing his nickname and the title of the book.
Critical reception
Newsround called the book "really exciting" and said "Once you've read page one you'll be hooked to Eoin Colfer's Books".
Sequel
In a September 27, 2007 interview Colfer admitted he had a rough idea for a second Half Moon book in which Moon goes undercover with a group of goths and listens to Kate Bush and Metallica. He said would find it interesting to expand on the history of Hazel - Moon's sister.
Television adaptation
In 2008, BBC announced a series called Half Moon Investigations to replace Grange Hill. A single 13 episode series later aired.
