Rain is used throughout the narrative as a metaphoric representation of trouble and/or difficulty for Martyn. The connection is established in "Part 1, Wednesday, Section 2" when Martyn's realization that he is in "a sweet mess" is juxtaposed with the beginnings of a rainstorm. Here it's interesting to note that within moments of this juxtaposition, Alex arrives...

Plot Summary
Fourteen-year-old Martyn Pig is faced with some difficult choices in the aftermath of his brutal father's accidental death. While he is relieved at being free from Dad's tyranny, Marty also feels a degree of guilt over that relief and conflicting emotions about the the flirtatious and mysterious quality of the girl next door; the blackmailing machinations of her repulsive boyfriend; and the sudden appearance of a large sum of money. As the narrative explores the moral ambiguities associated with both Martyn' situation and his decisions, it also explores issues related to becoming an adult and the allure of freedom.
Martyn begins the first-person narrative of his story with a description of some of the difficulties that he has faced, including having a drunken and violent father; a mother who disappeared from his life early on because of that father; and a name that has triggered series of taunts and endless bullying. He also sets the stage for the story that follows, describing the events that occur within the week before Christmas Day and an important participant in those events - Alex, the girl next door that Martyn has a crush on.
The actual story begins one night a week before Christmas. During a violent confrontation with Martyn, his drunken father accidentally falls and hits his head. Instead of calling the police, Martyn, afraid that he will believed to be responsible for his father's death and sent to prison, leaves the body where it fell. Later that night, when Alex comes by for one of her regular nightly visits, Martyn tells her what happened. Alex tries to convince him to call the police, but he refuses. The next day, Martyn discovers that his father has inherited sixty thousand dollars. He tells Alex, suggesting that if he goes to the police, there will be no way that he can collect any of the money and telling her that now she is as "guilty" as he is. Still convinced Martyn should call the police, Alex leaves, but returns later that day with her slimy boyfriend Dean, who reveals that he knows everything that happened and that he wants the entire sum of money in order to stay quiet. Martyn agrees to have the money for Dean by the following Monday, but concocts a plan to prevent that from actually happening.
After convincing Martyn's nosy Aunty Jean that Dad is still alive, on the Saturday before Christmas,