"U" Is for Undertow is the 21st novel in the "Alphabet" series of mystery novels by Sue Grafton. It features Kinsey Millhone, a private eye based in Santa Teresa, California. The novel, set in 1988, finds Kinsey investigating the disappearance of a 4-year-old girl in 1967 and the narrative weaves between both time periods. The novel has reached the top position on several best-seller lists.