Twelve-year-old Matthew Corbin hasn't left the house in weeks because of his severe OCD. To stave off boredom, he spies on the neighbors through the windows and fills notebooks with the details of their schedules. When Teddy Dawson, a toddler staying with his grandfather Mr. Charles in the house next door, goes missing, Matthew decides to solve the mystery, as he was the last person to see Teddy. He is helped by Melody Bird, a neighbor girl who communicates with him mostly via email.
The Goldfish Boy contains examples of:
- 13 Is Unlucky: Matthew is so superstitious that he refers to the number as "tenplusthree." If he sees it, he counts to 7 in his head, which neutralizes the number by making it 20.
- Baby's First Words: According to Mum, Matthew's first word was "Bum-bum," spoken at eighteen months.
- Creepy Child: Matthew, Melody, and Matthew's mum are all creeped out by Teddy's six-year-old sister Casey, who shows surprisingly little emotion at her brother's disappearance and is always playing with an ugly antique doll her mother gave her.
- Disappeared Dad: Melody's dad walked out to get away from the constant fights with her mother.
- First Friend: Matthew and his neighbor Jake Bishop were friends almost from birth, since their mothers were friends. They remained best friends for most of their school years, but when Jake was bullied for having eczema, Matthew failed to stick up for him. Jake never forgave him for that.
- Handshake Refusal: When Mr. Charles and Teddy first meet, Mr. Charles extends his hand and says, "Pleased to meet you, Teddy. I'm your granddad." Teddy just stares at the hand until Mr. Charles gives up.
- Hates Being Touched: Matthew, because people are germy.
- In-Series Nickname: "Goldfish Boy" is what Casey says to taunt Matthew. Jake picks it up too.
- It's All My Fault: Matthew blames himself for the death of his newborn brother Callum when he was seven. He had chicken pox and vomited on Mum. The baby was born and died a few days later.
- Kick the Dog: Before Teddy's disappearance, Matthew sees Mr. Charles dump a bucket of water on Nigel the cat, then try to kick Nigel. Matthew hates Nigel, who he sees as infested with germs, but he's still shocked by the cruelty.
- "The Reason You Suck" Speech: Jake offers to help with Matthew and Melody's investigation. Matthew replies, "You? Help? When have you ever wanted to help anyone besides yourself?" Jake shouts at Matthew for not defending him from classmates or their cruel gym teacher, even though they were supposed to be friends, and ends with "Forget it. I wouldn't want a friend like you anyway."
- Scars are Forever: The night Callum died, Matthew kept picking at a spot on his forehead, despite Penny's warnings that doing that would leave a scar. It did, and the scar serves as a reminder of his guilt.
- Spiteful Spit: While walking by Matthew's house, Jake stops to cough up a lump of phlegm onto the front walk.
- Terrified of Germs: This is the obsessive part of Matthew's OCD. Compulsions include washing his hands until they bleed, wearing latex gloves all the time, and obsessively cleaning everything in his bedroom and the bathroom.