The cover illustration features a boy in his after-school clothes surfing on a wave of Monster Blood.
Mark Corwin can't stop thinking about Monster Blood and what happened to his friend, Evan Ross last summer. It was so horrible. So terrifying. Too bad Mark's English teacher doesn't believe him. Now he's stuck working with the school janitor as punishment for not listening. Then Evan and his friend Andy come to help, and things go from bad to worse. Because they got a present to help him pay Mark's English teacher back. Then Mark accidentally eats something which is green and slimy and it's starting to grow...
Mark Corwin's no-imaginable teacher, Mr. Robert Crosby, is getting tired of the Monster Blood thing. Mark is always telling him stories about the Monster Blood and usually gets Mark into trouble. Evan Ross and Andy decide to help Mark strike back by sticking a bit of Monster Blood in one of Mr. Crosby's shoes. The Monster Blood is stuck on the shoe but he shakes his shoe, so Mark accidentally swallows some and soon begins to grow bigger and bigger, almost ending up trapped in the school. He narrowly escapes and stops growing, when he is several trees high. His clothes also grow with him, because they are splashed with some of the Monster Blood.
At first, he gets Kindergartener's kites out of trees and they call Mark "Super Mark", but he crushes a car. After more unintentional accidents, the police and fire department hound the enormous Mark, believing he is an alien. Fortunately, Evan's cousin, Kermit Majors quickly invents a compound which will make him shrink. Mark drinks the formula and (after a few unwanted results such as turning blue, getting hiccups, and growing feathers), he shrinks back to his normal size. The book ends with Mark, Evan, Andy, Kermit, The Police, and The Fire Department catching Mr. Crosby mixing a new Monster Blood known as the 12 Hour Trial Monster Blood.
Alternate Ending Edit
After Mark shrinks back to his normal size, he runs to get his backpack and leaves the school with Evan and Andy and Kermit. Then they find a new type of Monster Blood known as Vampirism Goo. Kermit thinks it's just a juice and drinks some. Then he stares at Mark, Evan, and Andy with red eyes and sharp-looking fangs and tells them he is really, really thirsty.
- First copies of this book shows the Monster Blood mixed the Purple Peanut Butter, Blue Monster Blood, 12 Hour Trial Monster Blood, and Vampirism Goo. However, it was changed for unknown reasons.