When eleven-year-old Piper chases a popcorn ball fifty years back in time, she meets Rosie, who greets Piper as an old friend. Although this is Piper's first visit to the past, it's far from the first time that Rosie has seen Piper. As Rosie explains to Piper, "It's time travel! Do you expect things to happen in order?"
The two girls swiftly become best friends. Together they eat cake, make snow angels, talk about books, slip back in time to witness a bootleggers' shootout, and learn how to fly during a lengthy trick-or-treating session on Halloween.
But Piper's sister Angela doesn't believe that Rosie exists - and soon Piper herself is worried that she won't be able to visit Rosie much longer. Piper, Rosie says, is always eleven during their visits... and soon Piper will turn twelve.