Nine-year-old Sergio is the only child of his middle-class parents, Anna and Paolo, but he may soon be joined by a sibling. Anna is pregnant and she doesn't want to bring the unborn baby to term, especially now that her marriage to Paolo is going through a rough patch. Sergio, though well adjusted, is a solitary boy whose introverted habits have earned him the nickname Lone Ranger. During a long summer vacation in the country, Sergio first fiercely resists the prospect of a sibling, then battles with a terrible sense of guilt when his imaginary brother fails to materialize. The impish figment of his hyperactive imagination refuses to disappear, however, and comically becomes an inseparable if annoying companion.