After Elastigirl hangs up in her plane, Kari plans to watch Jack-Jack play while she plays Mozart music, purportedly to stimulate his mind. Jack-Jack then teleports himself to the kitchen counter, and then the fridge. Kari tries to call Elastigirl again, but nobody answers. While she leaves a message, Jack-Jack sits on the ceiling. Kari then sees him teleporting himself from his crib to a high location. She goes downstairs and finds him floating around. Kari simply decides to show flash cards to Jack-Jack.
When Kari holds up a card with a campfire on it, Jack-Jack bursts into flames, but Kari puts him out in the bathtub. The next day, Kari is utterly exhausted but seems to have mastered anticipating his powers (armed with a fire extinguisher, mirror, etc.), when Syndrome comes to the door; Kari assumes he is a replacement babysitter, which Syndrome feeds into by hastily making up a story in which he falsely identifies himself as "Sitter" (his story being that the "S" on his costume stands for "Sitter" and that a "BS" for "Baby Sitter" was not appropriate for taking care of children, as BS can stand for a curse word known for it's abbreviation) and eagerly places Jack-Jack in his care, not, of course, realizing that this man is a supervillain with the intent of training Jack-Jack against his family (as stated in the finale of The Incredibles).
Rick Dicker asks Kari if she has told anyone, and she says that she has told her parents, but they thought she was joking. Kari then wishes she could forget the whole incident (she found it odd that Jack-Jack could "blow up", as she put it); to her fortune, and for the sake of the supers' anonymity, Rick proceeds to erase Kari's memory of it.
The short shows us that, adding to his lead, fire, and monster powers, Jack-Jack also has the ability to create portals in solid objects as he touches them, anti-gravity, and laser vision.