As Claude continues to tutor Rapha Jr., he systematically studies each family member. Rapha Sr. suffers business troubles with his Chinese business partners, Rapha Jr. struggles with feelings of inadequacy, and Esther sinks into depression over her meaningless home decorating. Germain actively coaches Claude on dramatic pacing: he tells Claude that a story requires conflict and a romantic triangle.
To keep his spot as Rapha's tutor, Claude forces Germain to steal an upcoming math exam from the school's safe. Claude feeds the answers to Rapha, cementing his position in the house. Germain, completely intoxicated by the fiction, complies—committing academic fraud.
Claude then accelerates the drama: he kisses Esther, instigating emotional chaos. Rapha Jr., realizing Claude has used him and suspects feelings between Claude and his mother, confronts Claude and attempts suicide by overdosing on pills. Esther discovers the draft pages of Claude’s writings describing her household with mockery, precipitating a final break in the Artole family; Esther and Rapha Sr. separate, and the home is put up for sale.
Having destroyed his first subject, Claude shifts his narrative focus to Germain’s own home. In the ultimate twist of manipulation, Claude visits Jeanne, reading her his work and seducing her emotionally and physically. Meanwhile, Germain's theft of the exam is discovered; he is summarily fired and disgraced. When Germain discovers that Jeanne is leaving him—and realizes Claude has conquered his own wife—Germain confronts Claude in the school auditorium and slaps him, only to be completely abandoned by Jeanne.
The Ending: Germain, now divorced, unemployed, and residing in a modest flat, sits on a park bench overlooking a vast, multi-windowed apartment building. Claude sits next to him. Despite everything that transpired, the mentor and the protégé are inextricably bound by their shared pathology: the need to observe and invent stories. As they look into various windows, speculating on the lives and secrets of strangers, Germain remarks that no house is impenetrable; there is always a way in.