An overweight, timid fifteen-year-old boy and his popular fourteen-year-old sister begin to overcome their guilt over their father's death and reconnect with each other and their emotionally-distant mother when they accompany her on a two-week speaking tour.

Melly and Anny Beth agree to participate in Project Turnabout, a scientific experiment in which they are given a shot that will make them grow younger, until they receive a second injection that will stop the aging process, but when other participants die after receiving the second shot, Melly and Anny Beth refuse to have the shot and set out to find someone to care for them when they are too young to do it themselves.

Fourteen-year-old Avery Armisted and sixteen-year-old Kayla Butts, once good friends, begrudgingly travel to Spain together for a summer vacation where they uncover a secret their families kept hidden from them their entire lives.

"It isn't natural for you to be younger than your great-grandchildren. We messed around with nature, and we shouldn't have. Melly and Anny Beth both lived normal lives throughout the 20th century. But in 2000, when they are old and ready to die, they are selected to participate in Project Turnabout and are given an injection to make them grow younger..."