Catherine Willows wants to return to the CSI team. She's at loose ends after leaving the casino business. Maxine suspects there's more going on than Catherine is saying. Her first case back is the murder of a dominatrix, Lynn Zobrist. The killer redecorated her sex dungeons with mirrors. He killed her with a mirror as well. Gina, the cleaning lady, says that Lynn had a kind side. New technician Beau figures out how to remove the mirrors without damaging the crime scene underneath. Penny determines some of Lynn's clients from her social media. There's too much blood at the scene to be from one person. Returning from training, Allie is going to try out her new molecular cartography skills to sort out the various substances at the scene. Serena talks to Dr. Auerbach, Lynn's longtime psychiatrist. The doctor is cagey, but her theory is that the killer will strike again. Interrogations of Lynn's clients yield nothing. Allie determines that Folsom and Serena are a couple. Catherine and Folsom find broken, burnt furniture in Lynn's fire pit, likely the piece missing from the dungeon. The tar-like substance Beau retrieves from the furniture leg turns out to be from a medicated cream. This leads them to Alan Herskovitz, a client who was scratching during interrogation. A search of his hoarder's apartment reveals that he's hunting people. Serena interrupts Herskovitz's stalking of Lamont Moore but he is armed and escapes. Lamont and Lynn were young victims of the same psychiatrist who used mirrors. Lynn wounded Herskovitz in their struggle and he's suffering from sepsis. He takes a hostage at the hospital where he's being treated. Herskovitz is another victim and says a voice chose him. Catherine's protege Grace has been missing for three weeks, which is why she came back to CSI.