A year has passed since Matthew said goodbye to his alien friend, and in the summer holidays he meets Albertine, a mathematical prodigy, with whom he discovers he can communicate telepathically. One day Chocky returns to warn Matthew that they are both in danger. When he returns to tell Albertine, he finds she has disappeared.

Matthew Gore is an intelligent boy chosen by a mysterious extraterrestrial visitor to be a source of information about life on Earth. As his schoolwork and artistic talent improve dramatically he arouses the suspicion of powerful groups who wish to tap into the amazing fund of knowledge to which he is now party.