Unless I'm using "backward" and "backwards" wrong, I can pluralize it when the subject is not plural and vice versa.
For example, correct me if I'm wrong:
If you watch your favourite movie backwards, what is the plot?
More than one movie.
If you watch your favourite movies backward, what are the plots?
I've never stopped to think about this before and am looking for some quick enlightenment as to its validity.
I've seen this question based on the same words but believe it's a slightly different context.