It's Matthew's birthday and the family have gone to London. They go on an open-top bus and visit the planetarium. Matthew explains that he wants to be an astronaut when he grows up. It is discovered that Matthew has been adopted. In Maths the following morning, a blue-white light appears and enters the classroom. It seems to be searching for someone. It lands on Matthew and Matthew immediately finds that he can solve the problems straight away, when he couldn't before. Matthew's parents notice his unusual behaviour as he starts to ask some unusual questions. Matthew has a fever and starts talking aloud. His parents discover Chocky, who seems to be an imaginary friend.
The next morning, Matthew's temperature has lowered and he returns to school. His mum is still concerned about him. At school, Matthew has a biology lesson and he asks his teacher, Miss Blayde, about reproduction in a single sex species. Later, his dad gets a new Citroen car. Everyone is excited, but Matthew stands next to the car in a terrible, distressed state. Chocky finds the car stupid and old-fashioned and hates it. At the next Maths lesson, Matthew starts using the binary code to count as that is what Chocky uses. He uses Y and N, for Yes and No, instead of 0 and 1. Matthew plays an electronic game with his friend Colin. When it is Matthew's go, Chocky is curious and she makes the screen explode.
The family go for a picnic beside a river. Chocky is interested about how boats float and Matthew finds a boat and investigates without the owners permission and without telling his parents. The boat owner thinks he has caught a thief and hands Matthew over to a policeman. His dad goes looking for Matthew and finds him with the policeman. Dad's old friend, Dr. Roy Landis comes to see Matthew to speak about Chocky. Landis discovers that Chocky is real and not imaginary. In bed, Matthew finally sees Chocky as a form of a swirly misty hologram
It is the summer holiday for Matthew and Polly. Matthew's mum is tidying up Matthew's bedroom when she finds some strange art work. Matthew admits painting these but in the eyes of Chocky. Matthew is miserable as Chocky has deserted him for his holiday. They meet up with Colin and his family. The children are playing near the river when a boat breaks free and starts to drift downstream. Emma screams which alerts Polly and Matthew. The jetty is destroyed by the impact and Polly and Matthew fall into the water.
Matthew saves Polly's life and Dad is shocked, because Matthew could not swim. Matthew explains that it was Chocky that made him swim. Colonel Summers who helped rescue him has put him forward for a medal. Matthew is asked about his experience by a stranger. He turns out to be a reporter and Matthew is in the newspaper and on the radio. When Matthew gave away the information, he spoke of a 'guardian angel'. Mum is not happy when she discovers that one of Matthew's strange paintings has been but forward for an art competition. It wins first prize. On Landis' request, Matthew is taken to see Sir William Thorbe. He examines Matthew, but afterwards Chocky suspects danger and leaves immediately.
The school day has ended and Matthew is kidnapped. There are appeals on television and everyone is searching for him. In Birmingham, Matthew is found with evidence of being injected and he believes that he has been involved in a car crash. Matthew is unaffected and at home, Chocky appears. She says that she would like to speak to his dad. She explains everything and proves that she is real. Chocky then disappears forever. Dad engraves the swimming medal with 'Chocky' for Matthew.
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.
Albertine is studying at Cambridge University where she is excelling herself. She wishes to research about cosmic energy, but she meets resistance from Professor Ferris who is fascinated about her theories. Albertine uses an illegal radio telescope to carry out research, but she is discovered by Dr Liddle, who doesn't enjoy her presence. Chocky returns to reveal that her people are prepared to reveal the secret of cosmic energy to the human race.
Matthew receives an art scholarship. Meanwhile, Chocky contacts other children from around the world who she wants to build some equipment that can harness cosmic energy. Albertine is joined by Mike and Su Lin who help her with her research, but she is later joined by the mysterious Paul.
The children start building the equipment based on Chocky's instructions, though Professor Draycott is strongly protesting. He is worried about funding and thinks that it is a waste of money. Draycott is not convinced that they are doing something worthwhile so he sets them a deadline.
It is the demonstration of the new equipment, but some of it has mysteriously disappeared and the research grant has ran out. But Meyer receives a visit from Mrs Gibson who claims to represent the Ministry of Science and she makes an offer to fund further research. Albertine and her father discover a sleepwalking Paul in the middle of the night and he is found to be tampering with the rebuilt equipment. It is revealed that Mrs Gibson's real name is Major Gibson. Major Gibson is working on behalf of an army general who has his own designs for the project.
Major Gibson watches closely as work continues on the next stage of the project. Paul runs away and Albertine tries to pursue him. He leads her to his mother's house where Albertine discovers that his mother is in fact Dr Liddle. Paul has been giving her the details of the project and she believes their proof. Gibson reveals to Meyer and the children that he wants to take over from them and use what they have so far.
Gibson steals a vital part of equipment which he puts into a crate in which Chocky and Paul are also trapped. The crate is moved to a military base. The children discover that Chocky has vanished and so they make contact telepathically with Matthew who returns to help them. Together they project their collective mind to Chocky's home planet across the universe. Another of Chocky's kind helps the children in their search for Paul and he visits Earth. Together they find Chocky and Paul trapped inside the crate at the military base. Gibson is arrested and Chocky returns to her home world. The team are left to continue their work to help the rest of the telepathically sensitive children around the world.
