Seventeen-year-old Maddie O'Neill Levine's tattooed, sassy but utterly lovable grandmother, Astrid, has never conformed to the standards of her Manhattan socialite circle, and has always been Maddie's go-to confidante. Even though Maddie and the rest of her family have learned to expect the unexpected, Gram still manages to shock them when she announces that she has booked them all on a secret death-with-dignity ship called the Wishwell. She has terminal cancer, and she's determined to leave the world in her own way, and give her family an unforgettable summer of dreams-come-true - and an understanding of who they are and how they've come to be - in the process.

Soon, Maddie is on the trip of a lifetime with her crazy family - parents, sibling, cousins, aunts and uncles. Aboard the ship, Maddie bonds with the other Wishwellians, each dealing with their own grief, and falls for Enzo, the son of the ship's owner, as they travel the globe. But despite the glamorous destinations, Maddie knows she is on the brink of losing Gram, and she struggles to find the strength to let go in a summer shaped by love, grief, and the power of forgiveness.