After Stan's sister, Shelly, is laid up in the hospital with chickenpox, Stan's, Kyle's, and Cartman's moms agree to have their sons spend time with Kenny (who has chickenpox) so they can get it now (as chickenpox isn't as dangerous in childhood as it is in adulthood). Meanwhile, Sheila (Kyle's mom) learns from Carol (Kenny's mom) that Gerald (Kyle's dad) and Stuart (Kenny's dad) used to be friends when they were teenagers and arranges for the men to reunite.
"Chickenpox" contains examples of:
- Double Take: Too late does Chef realize he told the kids how to best get revenge on their parents.
- Everybody Laughs Ending: Everyone laughs at Cartman's joke at the end, though they pause briefly when Kenny dies.
- Final Solution: Gerald tries to tell Kyle about the differences between the rich and the poor, by Kyle thinks he meant this, never mind that Kyle's family is Jewish and they should know that killing off a large group of people is a bad idea (The Holocaust, anyone?).
- Karma Houdini: Shelly's abuse of Stan throughout the episode when he visits her and when they're in the hospital together is completely ignored by Sharon when she's around.
- Kill the Poor: Gerald inadvertently gives Kyle this advice.
- Laser-Guided Karma: The parents trying to purposely give their kids chickenpox is countered by the kids giving them Herpes, which they feel they deserved.
- My God, What Have I Done?: Sharon and Sheila's reaction to their kids passing out from chickenpox.
- Gerald's reaction to reading Kyle's letter saying how he interpreted his advice.