The cinematic montage installation Obsession is unique with respect to its interaction method: how the story is told is not directed by any kind of conscious activity of the spectators, such as clicking interface elements with the mouse. Introducing the notion of enactive cinema the work emphasizes unconscious interaction between the cinema and the spectators as participants, referring to the idea of acting with and within an environment. In this solution the narrative flow is driven tacitly by the participants' heart rate and skin conductivity evolving in response to the events on the screen, and a detector of orientation in the installation space. The invitation to enact this way is very gentle. In the middle of the space there are five chairs to sit down. On the arm of each chair a red-lit shape of a hand attracts the spectator to place her hand on the built-in biosensors. Each participant's heart rate is continuously measured from her fingertip, and the emotional arousal is captured from her palm in terms of skin conductance. The spectators' common contribution is collected for the input to the software in charge of the narrative flow. This article concentrates on the explication of the narrative logic of this installation inspired by the Eisensteinian 'montage machine', and leaves aside a number of details of its complete setup.