Putnam, the future king of Raftworld, wants more than anything to prove himself. When the water in the Second World starts to become salty and his father won't do anything about it, Putnam sees his chance. He steals a boat and sneaks off toward the source of the salty water. He doesn't know he has a stowaway onboard, an island girl named Artie. As the two face uncertainty and danger in their shared adventure, an extraordinary friendship forms. Meanwhile, a hundred years in the past, Rayel, princess of Raftworld, flees to southern waters, escaping an arranged marriage, and foiling a plot to kill her father.

Told in alternating perspectives with Putnam and Artie traveling further and further into the uncharted southern sea--and Rayel, the key to the saltwater mystery, sailing the same sea in her own time--Putnam and Artie must put aside their differences and figure out why the sea is salty before it's too late.