Seventeen-year-old Maddie O'Neill Levine lives a charmed life, and is primed to spend the perfect pre-college summer with her best friends and young-at-heart socialite grandmother (also Maddie's closest confidante), tying up high school loose ends. Maddie's plans change the instant Gram announces that she is terminally ill and has booked the family on a secret "death with dignity" cruise ship so that she can leave the world in her own unconventional way - and give the O'Neill clan an unforgettable summer of dreams-come-true in the process.

Soon, Maddie is on the trip of a lifetime with her over-the-top family. As they travel the globe, Maddie bonds with other passengers and falls for Enzo, who is processing his own grief. But despite the laughter, headiness of first love, and excitement of glamorous destinations, Maddie knows she is on the brink of losing Gram. She struggles to find the strength to say good-bye in a whirlwind summer shaped by love, loss, and the power of forgiveness.