Joys little sister invites her boyfriend over to fulfill a little favor for him. Joy over hears this and waits for her sister and her boyfriend to head up stairs. She sneaks up stairs, peaking her phone into the room to get a few shots of her little sister blowing her boyfriend.
After her sisters boyfriend leaves, Joy confronts Diane and shows her the pictures. To keep her from showing their mother though, shell have to put on a pullup. This is all revenge for making fun of her for her bedwetting. Now her little sister will know what it feels to wear a diaper. But her punishment isn't over yet. Joy takes her to their playroom and tells her to sit in there and play.
Diane is upset and just wants the pullup off, but after a little while, she begins to enjoy playing like a baby. Shes completely regressed into an adult baby, just enjoying her time diapered and playing with toys. But it's tiring playing with toys, and poor baby Diana falls asleep.
Joy comes rushing in and wakes up her little sister because their parents are home and they need to hide that Diane is diapered ... Actually, not only is she diapered, but she wet herself in her sleep. Joy teases her, poking at the wet diaper and calling her little sister a bedwetter. But their mother knocking on the door makes them hurry. Diane wants to go change, but Joy wants her that their mother will see her and the only thing she can do is just walk downstairs with their mother in her wet pullup and eat dinner that way.
After dinner, Diane realizes her pullup leaked. Not just a little, but a lot. She rushes upstairs, terrified everyone saw her wet pants that are now see through. Joy follows her up and comforts her little sister. Feeling bad, she offers to delete the pictures and then helps clean her poor wet little sister up and goes and throws away her pullup.