At magic school, Piper and Paige orb in and start talking about a krychek demon, which is attacking other magical beings on behalf of the government. The demon is attacking the demons and comes after them, and proves highly resistant to both potions and Piper's powers. They manage to knock it out to take back to Murphy, but the demons attack before they can grab it and they're forced to orb out.
They get back to the manor and check the Book of Shadow, and find the picture of a krychek demon that doesn't match what they saw – the one at the school was much larger and more ferocious. Leo suggests Piper take a week off but she needs to talk to Murphy first, and they can't get hold of Billie. Phoebe tells Paige she got a call from Henry, and Piper and Phoebe go to check in with Murphy while Paige finds Billie. Billie is at the school and looks on as the krychek demon reverts back to a more normal demonic form, and the demons save him for their leader, Margoyle. Then the transformed demon burns up, much to the other demons' surprise. They spot Billie who tries to pass herself off as a student, but grabs one of the demons, Emrick, and demand he give her information about the demon that took her sister.
Paige answers the doorbell and finds Henry with a baby, Ramon, who will be going into foster care if he can't find the father. He wants Paige to look after the baby and she reluctantly accepts before he takes off.
Piper and Phoebe confront Murphy who claims he's as out of the loop and can't answer their questions. Phoebe reveals she doesn't believe in her premonition powers any more and that they gave up on her. Murphy demands they find the demon and get him back. Billie returns and she and Paige have words, but they're interrupted when Emrick up, now in the same mutated form. He scratches her before Paige can orb him to magic school.
Piper and Phoebe get back to find Billie cleaning up from the attack, and she fesses up about looking for info on her missing sister. They compare notes and then go to check on Paige and Ramon. Phoebe ends up with Ramon while they go to check out magic school. At the school, Margoyle shows up and reveals the plague is happening all over the underworld, and he wants to obtain the power to use it against the other overlord demons. The infected demon burns up like the other before they can get answers, and Margoyle destroys the demon who trying to get answers. Another demon, Tracka, suggests the sisters might know something and Margoyle decides to wait for them.
Billie is going through the Book of Shadows and asks Leo for advice as to whether she can be infected by a demonic device. He says no before he heads off to prepare for his trip with Piper. Paige confronts Henry to have him take the baby back and grabs some info to scrye for the father, while Phoebe passes Ramon off on Piper. She hears a noise downstairs and goes down to find that Billie has transformed into a similarly monstrous form, and Piper has to use her powers to take her out.
Paige returns and they contact the Elders while Paige watches over Billie. The Elder arrives and reveals the virus is affecting everyone throughout the magical community, good or evil, and they don't know where it originated. He reveals Billie doesn't have much time and the more powerful you are, the quicker the virus kills the infected.
Paige comes back to give Henry the information on Ramon's father, Ernesto, then is off again despite his offer to help. At the manor Billie goes into convulsions then transforms back. Paige orbs in, and her and Piper go to confront Murphy and threaten him unless he comes clean. He reveals that the government found a demon in the desert and captured it, then injected it with human blood as part of an experiment to create super-soldiers. An infected person escaped and is immune to the virus' effects, so its blood could provide a cure. Murphy gives them the guy's ID code bar, although it's been erased.
Phoebe is tending to Billie when Piper and Paige return and bring her up to speed. Phoebe concludes they have to infect themselves to have enough power to find and capture the original demon, while Tracka observes what's going on. They mix Billie's blood with their while Piper freezes Billie and Leo looks through the Book of Shadows. The sisters tell him what's going on as they transform into huge hulking demons and slap him around.
They start fighting among each other but manage to control themselves enough to use their enhanced power to scrye for the infected demon's location. They then orb to where the infected individual is and Tracka and the other demons arrive, and a fight breaks out. The bulked-up sisters trash the demons but Margoyle gets a blood sample before fleeing. The sisters get the antidote to the Elders and cure everyone, and Murphy decides the witches are a bit too…rude to work with.
Paige meets with Henry who is talking to Ernesto, who is ready to take Ramon back after Henry reassures him. Paige returns as Henry finds out she didn't contact Ernesto, and they share a moment. Phoebe gets another vision of her future with a daughter, and her future self confronts her in the vision and she tells present-Phoebe she's there for a reason, and that the vision will come true and she should have faith. Leo and Piper finally head off for their trip but not before Phoebe assures Piper she's okay.