Hannah, Compton, and Kira have been friends since medical school, reuniting once a year for a much-needed vacation. Just as they gather in Spain, an outbreak of a fast-spreading virus throws the world into chaos. When Compton returns to her job as an ER doctor in New York City, she finds a city changed beyond recognition. Hannah's career as an ob-gyn in San Diego is fulfilling but she's always longed for a child of her own. After years of trying, Hannah discovers she's expecting a baby just as the disease engulfs her city. Kira, an infectious disease doctor at the CDC in Atlanta, finds herself at the center of the American response to the terrifying new illness.