While his parents are abroad, Matthew travels to the country to stay with his aunt. There he meets a girl called Albertine who is a maths genius who has a very strict father. The pair soon discover that they can communicate telepathically, but they are unaware that sinister forces are observing them closely.

Matthew and Albertine become friends and Albertine begins to hear sounds in her mind. Arnold Meyer, her father, tries to keep her and Matthew apart, but without success. He fears that she will be distracted from her studies. They make models out of clay together and Albertine creates a model of Chocky without knowing it. It mirrors a painting done by Matthew a year earlier.

Luke, Aunt Cissie's 'gardener' spies for the mysterious Dr Deacon. He discovers the connection between Matthew's painting and Albertine's model. Matthew is warned by Chocky that by staying at his Aunt's house he is bringing danger to himself and to Albertine.

Matthew tells his Aunt that he has to leave and smashes the clay model. Albertine is due to be taken to London by her father for intelligence tests. These tests are to be carried out by Deacon, but Matthew warns her not to go and explains to her about Chocky. But soon, Albertine disappears in suspicious circumstances.

Albertine has been discovered in hiding and she is taken to the London clinic for her tests. She is assessed hypnotically by a doctor employed by Deacon. Deacon is interested in cosmic energy that Albertine knows about through her dealings with Chocky. He wishes to get this power and use it for his own ends. Matthew is guided by Chocky and he tries to get to the clinic and rescue Albertine.

Matthew finds Albertine, but Deacon discovers them both together. Matthew's father returns and goes to see Dr Landis with Meyer to seek help. Using telepathy, Matthew and Albertine make their escape from the clinic and are found by their parents. Chocky tells them both that right across the globe, there are others just like them.