Shelly gets chicken pox and has to be hospitalized. The doctor explains to Mrs. Marsh that it can be worse for children when they get older. She decides that Stan should get it while he's young, so she convinces the other parents to get all of them infected by sending Stan, Cartman, and Kyle to Kenny's house for a sleep over, since Kenny already has it. Meanwhile, Kyle overhears his mother on the phone talking about the plan and decides it's time to get even.