Rex and Hannah are accidentally transported to a parallel world, where the United States is a Fascist dictatorship. While there, they discover that world's version of the Charmed Ones, who have no idea of who they really are.
This story was inspired by the Classic Star Trek episode, Mirror Mirror, and the 1971 Doctor Who story, Inferno. Hannah even makes a reference to that Classic Star Trek episode in question.
The Halliwell family that appears in this story are the parallel versions of them, not the Reality Prime ones.