Oona is 18 on New Year's Eve 1982 just about to turn 19. She faints at midnight and wakes up in 2015 as a 51-year-old though still 19 age wise. She has to adjust to the leap and what she has missed in her life. The next NYE it's another leap out of order though this time to 1991 as 20-year-old/27 combination.
You will find yourself reading more than you expect in one sitting.