Warning: Wikipedia contains spoilers
The film is set in the United States in the 1920s and 1930s. The title character, Zelig (played by Woody Allen), is a man first noticed at a party by F. Scott Fitzgerald, who has the ability to turn into other people when surrounded by them. For example, if he is among doctors, he transforms into a doctor, if around overweight people, he quickly becomes heavy himself. Zelig is called the "human chameleon".
Dr. Eudora Fletcher (Mia Farrow) is a psychiatrist who wants to help this man with this strange disorder. With the use of hypnotic techniques, she discovers that Zelig aims for approval, so he changes to fit in. Dr. Fletcher's determination allows her to cure Zelig, but the cure is worst than the disease, as Zelig develops a personality then he becomes extremely intolerant with other people's opinion.
Suddenly Dr. Fletcher becomes aware that she is falling in love with Zelig. Both patient and doctor, with the media coverage of the case, are part of the popular culture of their time. However, fame is the main cause of their division; the same society that made Zelig a hero destroys him.
Zelig's illness strikes back, he tries to fit in once more. With the accusations of women that claim to be married with him, he escapes where nobody can find him. Nevertheless, Dr. Fletcher doesn't give up on Zelig, and finds him in Germany before World War II inside the Nazi party. Together they escape in an airplane while being chased by the Nazis. After the adventure they experienced in Germany, they return to the USA as heroes.
Zelig used a very innovative and distinctive method to create the mockumentary feeling of this movie. For the film, Allen took real newsreel footage from the 1920's and 30's and inserted himself and other actors into the footage via bluescreen technology. Allen and his cinematographer even crinkled and scratched the negative to make the finished product look more like vintage footage. This was some time before digital film making technology made such techniques more easy; the concept was later used in such films as Forrest Gump and various television commercials.