This film tells the story of a successful writer called Harry Block, played by Allen himself, who draws inspiration from people he knows in real-life, and from events that happened to him, sometimes causing these people to become alienated from him as a result.
Roberta is a bored suburban housewife who is fascinated with a woman, Susan, she only knows about by reading messages to and from her in the personals section of the newspaper. This fascination reaches a peak when an ad with the headline "Desperately Seeking Susan" proposes a rendezvous. Roberta goes too, and in a series of events involving amnesia and mistaken identity, steps into Susan's life.