Seven has one plot—who the heck is killing people that commit the seven deadly sins, and why? As a result, it's pretty linear, even segmenting the film into days—seven of 'em, of course.
There is no narrator, although, strangely, Somerset chimes in with a couple of lines at the very end of the film, delivering an Ernest Hemingway quote: "'The world is a fine place and worth fighting for.' I agree with the second part."
This gives the bleak ending a glimmer of hope, but since there is no narrator throughout the movie, it feels tacked on. Any hope in this movie is an afterthought.