In the summer of 1973, a mysterious infection spurs women across the United States to leave their homes and migrate toward the Pacific Ocean. At a loss for what causes the infection, how it's spread, or what it may ultimately cause women to do, the public refers to the 700,000 women who go missing simply as 'the westward women'.

Three women are caught in the infection's riptide. Cautious twenty-two-year-old Aimee goes in search of her enigmatic, newly infected childhood friend, only to find herself haunted by the voices of other westward women everywhere she goes. Meanwhile, Eve, an ambitious young journalist, is determined to prove her worth by writing a story about a man she hears is collecting infected women across the country. Finally, infected sixteen-year-old Teenie, still grieving from the loss of her sister, begins traveling with a man known only as the Piper, who claims to protect girls like her...