The story begins with Dinah Glass, an eleven-year-old girl who has been living in a children's home for most of her life, being fostered by the Hunter family. The Hunters have two sons, Lloyd and Harvey, who are unhappy about having a foster sister, particularly Lloyd, who feels that Dinah will disrupt their lives. The family dynamics are strained from the start as Lloyd is openly hostile towards Dinah, and Harvey is indifferent but friendlyer.Dinah is quiet, polite, and emotionally detached, keeping to herself and not trying to connect with her new family. She soon discovers that she will be attending the same school as Lloyd and Harvey, though the boys seem anxious about it, especially about Dinah starting there.

When Dinah begins attending her new school St Champions, she quickly notices strange behavior among the students. All the children seem unnervingly well-behaved, robotic, and compliant. They perform tasks with military precision, follow rules rigidly, and never question authority. At morning assemblies, the students appear entranced, and Dinah soon learns that they are all under the control of the school's mysterious Headmaster, who exerts a hypnotic influence over them.

The school prefects, who act as the Headmaster's enforcers, ensure that all students comply with the school rules. The atmosphere at the school is one of fear and strict discipline, and the students are terrified of breaking any rules. Dinah meets the Headmaster for the first time and finds him an unsettling figure. He is a tall, pale man with strange, hypnotic eyes hidden behind dark glasses. When Dinah is called into his office, she experiences his power firsthand. He hypnotizes her without her realizing it, leaving her with memory gaps and strange behavior afterward. After their meeting, Dinah begins to say things she doesn’t truly believe, like praising the Headmaster as a “marvelous man” and calling the school the “best she has ever been to,” despite her growing suspicions.

Dinah cannot remember large portions of her day, particularly after attending the school’s assemblies. The Headmaster seems to have an unusual power over the entire school, and Dinah begins to realize that the students are hypnotized during assemblies, turning them into obedient followers of the Headmaster’s will. On the other hand, Lloyd and Harvey, along with a few other students, have managed to resist the Headmaster’s hypnotic control. These children, known as the "Normals," keep a low profile and avoid attending assemblies to stay out of the Headmaster’s influence. Lloyd is particularly protective of Harvey, as the younger boy is more vulnerable to the Headmaster’s power.

Initially, Lloyd and Harvey distrust Dinah, thinking she might fall under the Headmaster’s spell or betray them. Lloyd is especially wary of Dinah, convinced that she will become another puppet of the Headmaster. However, Dinah’s curiosity and intelligence push her to investigate what’s really going on, and she eventually becomes an ally to the brothers. As Dinah becomes more aware of the Headmaster’s hypnotic power, she decides to resist his control. She uses her intellect and determination to start piecing together the puzzle of what’s happening. Through a mirror technique (where she reflects on her thoughts by talking to her reflection), Dinah comes to the conclusion that fear is controlling the students and that she must act against it. Dinah decides that the only way to uncover the truth is to deliberately break the rules and be “naughty,” something she would normally avoid. This decision marks a turning point in her character as she takes an active role in resisting the Headmaster’s power.

Dinah is invited to join Lloyd, Harvey, and a few other students's secret group called SPLAT (Society for the Protection of our Lives Against Them) which is designed to resist the Headmaster’s control and uncover the truth about his methods. SPLAT operates in secrecy, trying to avoid the prefects and the Headmaster’s watchful eyes. They start investigating the Headmaster’s plan, suspecting that he has a larger, more sinister agenda than simply controlling the school.

The group becomes suspicious of the "Great School Quiz," a national competition in which the school is competing. They believe the quiz might be linked to the Headmaster’s broader plan for control, and they begin to plot ways to disrupt it.Dinah and SPLAT discover that the Headmaster’s ultimate goal is to use his mind control techniques to extend his influence beyond the school. By winning the Great School Quiz, he plans to broadcast his hypnotic power to the entire country through television, controlling the minds of millions of people.