In early 1960s New York City, concert pianist Henry Orient (Peter Sellers) pursues an affair with a married woman, Stella Dunnworthy (Paula Prentiss), while two adolescent private-school girls, Valerie "Val" Boyd (Tippy Walker) and Marian "Gil" Gilbert (Merrie Spaeth), stalk him and write their fantasies about him in a diary. Orient's paranoia leads him to believe that the two girls, who seem to pop up everywhere he goes, are spies sent by his would-be mistress's husband.