Piper and Leo prepare for a ski trip. Henry asks Paige to look for a parolee's baby while he tracks down his father. Phoebe grows disillusioned with her life and her powers. When Agent Murphy asks Piper and Phoebe to track down a Krychek demon, they find the usually inoffensive demon enraged and completely disfigured, and later Billie sees the Krychek die and another demon undergo a similar mutation. It turns out a virus is causing this, and Billie gets infected.
- Despair Event Horizon: Phoebe comes to believe her premonitions of having children one day will never come true and loses faith in her powers.
- Freudian Excuse: Henry wants to prevent the baby from going to a foster home because he was in one as a child, and it wasn't a happy experience.
- Hulking Out: The infected become more muscular, demonic looking, and powerful. The term is even lampshaded.
- My Future Self and Me: Phoebe's last premonition takes her six years into the future, where she talks to herself about her doubts over ever having a child.
- Parental Abandonment: The baby's mother is going to prison; the father is stable, but he doesn't even know about the kid. Henry is struggling to find the man, and Paige ultimately resorts to magic.
- Patient Zero: The demon known as Patient X. He was immune, but he became a carrier.
- Race Against the Clock: The virus is terminal; the more powerful the infected, the faster they die. The sisters go into overdrive to fix this after Billie is infected.
- The Virus: The overriding crisis. Onscreen victims are shown being exposed to it by cuts and bites. Agent Murphy later explains that agents found an injured demon and studied it, including injecting blood as part of an attempt to remove its powers. This created the virus, and it spread when the demon escaped captivity.