Bradley is, without a doubt, the most annoying character who ever appeared in one of these books. Our main character, Matt, is the star of the school swimming team and the biggest science nerd in school. Bradley is his next door neighbor. Though he doesn't like him and doesn't want him around, his mom keeps letting him in their house. The steals Matt's favorite clothes, eats his breakfast, and is just an asshat. When he tells him about his idea to build a computerized birdhouse, Bradley steals the plans and shows them to their teacher as his idea for the science fair.
One day on the way to school, Bradley picks up a rock and throws it at the stop sign. It bounces off and smashes into the windshield of their principal. Bradley runs off and Matt takes the blame, with the principal warning him that he needs to act right with the big swim meet coming up.
At school, Bradley cheats off him on a test. When their teacher catches them, he claims that Matt did it without him asking. They both fail the test, and the teacher tells Matt that it was nice of him to help his friend but that he now has two strikes. One more strike means he'll get suspended. That strike almost comes when he attacks Bradley and a teacher catches them fighting. Bradley then claims they were just playing and acts like Matt should kiss his feet for getting him out of trouble.
Matt then catches Bradley using his computer without his permission and runs him off. He finds his computer frozen on a website selling Monster Blood. The next day, two men in black show up on his doorstep. They tell him that they need back what he ordered, there's a short scuffle, and Matt manages to run away. Once the Monster Blood arrives in the mail, all hell breaks loose.
Even though he's an athlete, Matt is also a nerd who grows ivy in his room. He accidentally gets some of the Monster Blood on the plant. When he wakes in the middle of the night, he sees a man in his room. It's actually just the plant growing because it got Monster Blood on it. Matt also gets a little of it on his skin too, right before the big meet.
He immediately notices that he has more muscles and feels stronger. When he jumps in the water, he starts growing until he's more than six foot tall. This causes him to win the meet and break the national record. Unfortunately, he keeps growing and has to sneak out of the pool and into the locker room. His clothes won't fit and people keep coming and going, so he has to make a decision fast.
Thinking that water made him grow faster, he tries standing under the hairdryers to shrink himself. That doesn't work, so he sneaks out of the locker room and runs home. He runs into the guys from before, but they're actually looking for something else Bradley ordered off his computer. He goes to Bradley for help, but there's nothing he can really do. They find the instructions for the Monster Blood and learn it was just a 12-hour sample. He ends up going back to normal, but his plant remains big.
Suddenly it's the day of the science fair. Matt presents his birdhouse, and his teacher basically tells him that he won but that they still need to look at the other entries. Just as he's about to win, Bradley runs in with Matt's plant and wins the award after claiming he did an experiment with light waves. Matt considers telling the teacher that it's his plant, but then he sees the plant slowly wrapping itself around Bradley.
Lesson of this one? I really, really, really dislike Bradley.
HorrorLand Bonus Story:
Matt got an invitation to HorrorLand and ran into Billy and his sister Sheena. Sheena became invisible, and they found a weird mirror that showed two missing girls in it. They find two detectives and ask for their help. The detectives go to the mirror and find it gone, but they invite the kids to a secret room, which actually looks like some creepy laboratory.
After whispering between them, they make Billy get in an x-ray machine. They keep talking about finding something that's missing on the kids. Matt realizes that they mean the key card a Horror gave him in another book. He goes through the machine, they find nothing, and the kids run away. It turns out that Matt gave the card to Sheena, which turned it invisible. She hands it back to him, and he unlocks a door that lets them escape the two guys.
After running wild through the park, they come across a little store. Billy picks up a can of Monster Blood, Matt freaks out, and he drops it on the ground. It comes to life and expands, pulling him into it. Sheena tries to help her brother, and it pulls her in too. Byron, the Horror who gave Matt the card, pops up. He first says he can't help, but he then pulls out a mirror that stops the blood from taking over the kids.
Billy walks away free and clear, but there is no trace of Sheena. Other horrors show up and take Byron away. As he screams about how they can't do this to him, Matt finds a broken piece of glass on the ground. Though there's no sign of Sheena, it does show the two missing girls screaming in horror with a carousel on fire behind them.