The Intruder is based on D'Annunzio's book, The Innocent', transferred to the screen with minimal changes by Visconti. Set in the late 19th century, the plot tells of Tullio Hermil (Giancarlo Giannini), who, after a period of infidelity during which he has been the lover of two of his wife's friends, decides to rekindle the love that once existed between him and Giuliana his wife (Laura Antonelli). However, he is worried that his wife has not remained faithful: she has had an affair with the writer, Filippo Arborio whose child she is now expecting. Tullio, upset on learning of this, starts to hate the unborn innocent child, and think he could only win back Giuliana if the child were to die. By the time the child is born, Tullio has made his decision to kill it. At Christmas Eve, with the whole household at midnight mass, Tullio takes the child from its cradle, opens the window and exposes the infant to the icy winter winds. A few days later, the baby dies. Cynically, Tullio attends the funeral, then returns to Rome with Guiliana. Here, Tullio meets his former lover, Teresa Ruffo, to whom he confides the murder of the child. Teresa explains that a healthy child would be able to withstand the exposure to the cold, and that the infant would have not lived long in any case. But her consolation doesn't diminish Tullio's ever increasing remorse.