In 1984, fifteen-year-old Celestine Tolland and her parents were part of a crew of explorers who set off into space. Months after their departure, a virus ravaged the crew of the Phaeacia, killing nearly everyone aboard. The few survivors finally returned to Earth after nearly two years. However, because they were traveling near the speed of light-causing an effect known as time dilation-more than thirty years have passed on Earth. Celestine, who had finished her freshman year in high school in 1984, returns to Earth to begin her senior year in 2022. She is only seventeen, while her former classmates are all nearly fifty years old.

Back in her hometown, Celestine faces the same trials that every teen girl faces, but there are many new challenges. In addition to making friends, finding romance, and dealing with mean girls, she has to learn how to navigate social media and the internet, and to avoid suspicious members of the public who fear she is contagious. She also has to deal with survivor's guilt, living with her foster mother (who also happens to be her childhood friend), therapy, fear of a government conspiracy, and fear of falling ill...