The Whisperer in Darkness

For the first two acts, the plot essentially follows the lines of the short story. However, in the third act of the film, the protagonist, Wilmarth, uncovers an attempt by collaborators (or cultists) to open a gateway between Yuggoth and Earth. He foils the plot with the help of an added character, Hannah, the child of one of the collaborators. His escape, however, is unsuccessful and at the end of the film the audience discovers that Wilmarth has been narrating from a machine attached to the cylinder in which his brain now resides. This differs from the original story in which Wilmarth flees in the middle of the night and safely returns to Arkham. (Source : Wikipédia)