"Donor Unknown" follows the story of JoEllen Marsh, 20, as she searches for the sperm donor father she only knows as Donor 150. JoEllen grew up in Pennsylvania with two mothers, and a burning curiosity to know more about her anonymous donor father. When JoEllen discovers a unique online registry that connects donor-conceived children, she manages to track down a half-sister in New York. "The New York Times" picks up the story, which falls into the hands of Jeffrey Harrison, living alone with four dogs and a pigeon in a broken-down RV in a Venice Beach car park. In the 1980s, Jeffrey supplemented his meagre income by becoming a sperm donor at California Cryobank. His number was Donor 150.