A motion picture synopsis is a simplified version of a screenplay, containing the essence of the story to be told in a brief, but simultaneosly concise way, also mentioning in general terms the events that will contribute to the unfolding of the subsequent action.
The synopsis must contain certain narrative elements as time, space, the characters, and a general overview of the story which binds them all together. Characters and their actions are established in a summarized writing structure which usually includes the beggining, middle and end of the story. The synopsis must be closed, in a sense that it must reveal the ending of the story and it's subsequent conclusion climax, for the pitching effort to achieve a sense of purpose on it's readers.
The key moments usually are:
The conclusion of the different actions and happenings, will give an insight if the story will have a favorable or unpleasant ending for any of the story's characters. The adding of a certain sense of tension in the unfolding of the events is always advised as a way to add more value to the idea being pitched.
Even in a short version, a synopsis must contain all of the main aspects of the narrative, in a way that provides it's reader with a general idea of how the story will go on. It's a pitching tool used to show a narrative's value in a few lines or pages, and it's usually determinant in the decision of adopting the story there shown for a future film adaptation.