Many children have imaginary friends but one father becomes rather concerned that his son, Matthew, is a bit old to have one. His concerns deepen as his son becomes increasingly distressed and blames it on arguments with this unseen companion. As the story unfolds, it becomes clear that the friend is far from imaginary, but is an alien consciousness communicating with Matthew's mind—a fact that is of intense interest to shadowy government forces. Chocky reveals that it is a scout sent from its home planet and this attracts investigation from the government.