This documentary focuses on elements of Hitchcock's style found in Blackmail, Hitchcock's and England's first talking film, and relates it to other films that Hitchcock made both before and after Blackmail.
The plot in Blackmail is pretty simple - perhaps that is why it was chosen as an example? In it, a girl has an argument with her cop boyfriend and ends up in the apartment of an artist who attempts to rape her. She fights back and ends up stabbing him to death with a bread knife. She's observed leaving the scene, and is blackmailed as a result.
By analyzing different scenes in Blackmail the narrator talks about common themes in Hitchcock films - "The Blonde", the importance of food, landmarks, artwork, etc.
Another interesting thing that this documentary does is compare the silent and sound versions of the breakfast table scene in Blackmail. In the sound version, Alice jumps at every mention the neighbor makes of the word "knife". She's discussing the murder that Alice committed, and you can't hear anything else she's saying other than the word "knife" as the camera focuses on a traumatized Alice. The silent version can't do anything nearly as artistic with this scene.
I guess one of the things I took away from this documentary that really was never directly mentioned is that, from the beginning, Hitchcock really understood how to direct talking film. That may not seem like much until you look at the early sound work of other directors, including many prominent ones such as John Ford, who seemed to forget, at least temporarily, everything they ever knew about the art of motion picture making when confronted with sound in film.