Eleven-year-old Nate and his mum hide out in a run-down cottage in the woods to escape from Gary, Nate's emotionally abusive stepfather. Just as they're getting settled in, Nate's mum disappears, leaving him all alone. As Nate tries to survive on his own, his childhood Imaginary Friend Sam suddenly reappears, saying Nate needs him again. Shortly after that, he meets a girl named Kitty, who's hunting for treasure on the grounds of the nearby mansion.
The Light Jar contains examples of:
- Abandoned Area: The mansion, where Kitty claims to live, hasn't been inhabited in fifty years. Most of the windows are boarded up, and the ceiling has collapsed in several places. Nate's mum went in to look for firewood but fell through a hole in the floor. Nate finds her, injured and dehydrated but alive, when he goes into the mansion to look for Kitty.
- But Now I Must Go: Imaginary friends leave when they aren't needed anymore. Sam leaves after Nate's mum is found, even though Nate begs him to stay.
- Calling Parents by Their Name: Kitty refers to her dad as James. Nate thinks it's weird, but thinks it's because she's posh. It's actually because James isn't really her father.
- Disappeared Dad: When Nate was six, his dad moved to New York City to be with his American girlfriend, Carrie. He used to say he was going to visit, and then cancel because he was too busy. He did visit a year later, but Nate was so angry that he insulted him and told him how much better Gary was. They haven't seen each other since.
- Handshake Refusal: Kitty sticks out her hand when she introduces herself to Nate. Nate has never shaken hands with another kid, so instead he folds his arms and nods.
- The Hermit: William, the cottage's previous owner, rarely left the cottage or spoke to anyone in the years leading up to his death.
- Horrible Housing: William died just a few months ago, so Nate's mum expects the cottage to be in decent shape. Instead, they find that it hasn't been maintained in years. The paint's worn off, there's a hole in the window, the garden is a jungle of weeds, and a chicken has moved in and covered the sofa with droppings.
- Identity Amnesia: Kitty has been wandering the grounds for so long, she's forgotten she's an imaginary friend. She tells Nate that she's the Impoverished Patrician daughter of Charlotte's older brother James, and that her family lives in the few usable rooms in the abandoned mansion. By the end of the book, she's remembered who she is, and she says goodbye to Charlotte's memory and goes off in search of another child.
- Idiot Hair: In a flashback, Nate's mum unsuccessfully tries to smooth a tuft of hair on his head in preparation for Gary moving in.
- Laughing at Your Own Jokes: As Kitty and Nate walk though a Hedge Maze, Kitty says, "William has left something a-maze-ing, I just know it. Get it? A-maze-ing?" and laughs.
- Little Known Facts: Nate's favorite book, Freaky Things to Freak You Out, is full of information like "A man from Brazil has a pet maggot living in his eyelid" and "A rhesus macaque named Mike delivers the mail to the Himalayan village of Konapanthi every day." Nate's mum tells him the book is full of lies, and he gradually realizes she's telling the truth. Although it does help him solve two of the riddles by teaching him what an ice house is and how to identify the Orion constellation.
- Mess of Woe: After Nate's parents split up, his mum got really thin and stopped doing anything around the house. Her parents used to come over to make sure Nate's uniform was clean and there was food in the cupboards.
- Minor Living Alone: Nate tries to hide the fact that his parents aren't around.
- Parent Never Came Back from the Store: On their second day in the cottage, Nate's mum drives to the nearest supermarket. Nate is afraid Gary will find her if she leaves, but she points out that there's hardly any food in the cottage and promises to bring back a pizza and a chocolate cake. She never returns.
- Parent with New Paramour: At first Gary seemed like a type 2. Nate was afraid he'd monopolize his mum's attention, but he seemed really nice and always included Nate when they went out. His cruel, controlling personality only became apparent after he moved in.
- The Reveal: Kitty is an imaginary friend, just like Sam. She was the friend of Charlotte, a six-year-old girl who used to live in the mansion, until Charlotte got lost in the Hedge Maze and froze to death.
- Riddle Me This: The treasure hunt has rhyming clues written by William.
- Say My Name: Nate yells Kitty's name as he runs around the mansion looking for her.
- Security Blanket: Mrs. Ellie-Fant, a stuffed toy Nate's had since he was a baby. He brings her with him when he and his mum run away.
- Surprise Party: On Nate's twelfth birthday, Sam invites a bunch of his fellow imaginary friends to the cottage. They yell "SURPRISE!" when Nate gets downstairs.
- Taking the Kids: Nate's mum wakes him in the middle of the night, telling to pack so they can go on holiday. They drive off in Gary's car hours before he's due to get back from a business trip.
