This article takes a close look at an element that permeates the screenwriting process from initial idea to finished film, and suggests that at the beginning of the creative process there is a core or budding idea that takes gradual shape as the screenplay develops into the screen version. This element is neither detectable nor does it give the work unity at the beginning of the writing process, but results from such process. As the story develops, the authors (screenwriter, director, producer) negotiate about that intangible "something" they do not wish to lose. After a theoretical overview of Aristotle's notion of the poetic myth understood as the soul of the story and mimesis of action (Poetics, 1450a 9,16-18, 22, 32-35, et. al.), and Ian W. Macdonald's notion of screen idea (2013) seen as the goal at which a screen project aims, this essay presents a case study of the Latin-American film,No (Chile, 2012), written by Pedro Peirano, produced by Juan de Dios Larrain and directed by Pablo Larrain. The methodology combines narratological and thematic tools. As a result, the study shows how the screen idea can be tracked down by observing the transformation arc of some of the characters in the plot, and explains why the plot undergoes modifications throughout the different stages in the development of the film.