Three years after inventor Wayne Szalinski accidentally shrunk his and his next door neighbor's kids, his family have moved to Nevada and have welcomed a new son, mischievous two-year old Adam. One day, Wayne's wife, Diane, leaves with their daughter, Amy, to help her settle in her dorm at college, leaving Wayne to look after Adam and their teenage son, Nick, who struggles with puberty. He has developed a crush on Mandy Park, who Wayne later arranges to babysit Adam. The next day, Wayne takes Nick and Adam to Sterling Labs, where he has constructed an advanced derivative of his shrink ray which could make objects grow. He tests it out on Adam's favorite toy, Big Bunny. However, when his and Nick's backs are turned, Adam attempts to retrieve it and is zapped by the machine, which appears to short circuit and not enlarge the targeted object.