How restricted are parodies?

I'm thinking about making a serious parody of a show that would be rated M(originally a G). The parody would change a few ideas around, add new ones, and become a darker version of the original. The story is incredibly deep/partly disturbing but nothing psychological/gory(hard to explain think Men in Black). I read "fair use" and the categories of types of parody and it fits under the "comment on" one. BUT! I'm not 100% sure whats going on. Any input would be nice.