16-year-old Danny was living a great life. He was very popular at school, and even has a hot girlfriend. The only flaw was his bratty little sister Mary. She spends her spare time pulling pranks on him and causing trouble while blaming him to the parents, which they always believe her. One day, she goes too far with the fountain.
It was a typical day for both Danny and Mary (Danny at the high school, Mary at the middle school), until at lunch, when Mary hears rumors about the mysterious fountain that makes people younger. The thirteen-year-old then gets struck with an idea about her next prank on Danny.
Immediately after school, Mary grabs an empty water bottle, and fills it with the fountain water. She then hides it from Danny as she plans to spill some water on him, which will make him younger. As Danny is about to leave for his date, Mary decided to let her plan in motion, but Danny already left. Her plan B is to spill the water on him when he gets to bed.
Danny returns from his date and immediately went to bed, unaware of Mary's prank. In the middle of the night, Mary sneaks in to Danny's room, opens the bottle, and positions it so the water spills in to Danny's mouth. When she spills some water in Danny's mouth, Danny wakes up, and Mary immediately storms back into her room. Danny then gets up to chase Mary when he notices that his room is slightly larger. He then looks in the mirror, only to see the reflection of a boy roughly the same age as Mary, and smaller as well.
"Hey Danny", Mary tells him, "You know the water I just spilt in your mouth? That was from the mysterious fountain outside of school, and it just made you my age". "You mean that you..." Danny asks in complete shock,"TURNED ME INTO A KID!?". "Calm down Danny", Mary responds, "Just for that, how about I make you a little younger". "What? NO!" Danny yells to Mary, "DON"T DO THIS TO ME! I DON"T WANT TO BE A LITTLE BOY!!" But by the time he yelled it out, Mary already shoved more of the water into Danny's mouth, making him turn back into a nine-year-old boy.