On The Magicians Season 4 Episode 7, "The Side Effect," Penny is giving a newcomer a lesson on why where you shelve a book matters. Of course, this is The Underworld branch of The Library, and the books aren't just any old book. They're the stories of our favorite characters, and Penny has decided to spotlight three "side characters" to help newcomer Derek understand the importance of what they do.
The first story is Zelda's, yes The Librarian. She's started seeing visions of her dead daughter and is convinced that she must be trapped in the mirror universe. While she's on a personal quest to save Harriet, the other Librarians are implementing a plan to set up trackers to try to find Alice. The result of which will have devastating consequences for hedgewitches.
Zelda's story is entwined with Kady's, who on her off hours has been putting in time trying to solve the cases her alter ego was working. Rent is due and Kady needs to hit the black market to find the items she needs, but while she's there she finds that hedgewitches are dying due to enchanted Deweys that are interfering with their personal enchantments.
Meanwhile, in Fillory, Fen is having precognitive dreams that are predicting things that are going to happen. With the help of Josh she is able to lucid dream and figures out what might be behind the dreams.