The title of Scoob! Children's Movie is a spoof on the cartoon "Scooby Doo", in which "Doo" and his friends go around solving mysteries with the help of their mystery-solving pals.
The story begins when James Ritt (Colin De La Hie) meets up with his friend Scooby (Simon Darby) in his room. While they talk, the camera focuses on a remote control and then flashes to Scooby sitting at a large table eating a plate of cereal.
When James picks up the remote control, the movie then switches to the table where the kids are eating. Scooby's house is seen sitting on the table, while Mr. Black (Liam Barr) is sitting at the table eating breakfast. It then shows James taking Scooby out to his car for Scooby's first day as a driver.
James and Scooby then talk in the car, which is cut short when James notices a spider on his face and rolls over, causing a sign to pop up on the dashboard reading "Spider Face! Stop Now!"
With that, the title of Scoob! Children's Movie ends as it cuts to the scene of Scooby jumping into the air after his first day at school. The camera pans to the classroom where Scooby and the other kids are having their "first lesson" as "your teacher".
James gets distracted, and accidentally bumps into the teacher. As the camera goes back to the desk, we see Mr. Black actually sits there eating breakfast and looking bored. As he looks up from his cereal, Mr. Black grabs his glasses off the table and drops them onto the floor.
When Mr. Black looks back up, James walks into the classroom, with the camera panning down from James' feet to reveal a gigantic spider sitting behind the teacher. As the spider leaps at James, the camera cuts to the teacher's head getting swallowed by the spider.
The ending of Scoob! Children's Movie then reveals that James is actually in the hospital for an attack by a spider bite, and that all the kids were killed by the spider and the killer. At the end of the movie, a caption appears on the screen reading "Toby Alder's Scooby Doo"