The film stars Joan Fontaine, Louis Jourdan, Mady Christians, and Marcel Journet (actor) [fr].

In 1992, Letter from an Unknown Woman was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".

[4] In early twentieth century Vienna, a man named Stefan Brand returns home; he is due to duel against an opponent the next morning but plans to skip town and avoid it.

His mute butler hands him a letter, addressed to him by a woman who claims that she may be dead by the time he receives it, which Stefan begins reading.

Many years earlier, Lisa, a teenager living in an apartment building, becomes infatuated by Stefan, a new tenant and concert pianist at the time.

She stays up late to hear him practising, sneaks into his apartment while he is out, admires him from a distance, and seeks to dress better and act gracefully in the hope of becoming worthy of his attentions.

Still in shock, Stefan leaves his building and sees the ghostly image of a teenage Lisa open the door for him - a memory of their first encounter.

The film renames him Stefan Brand (referencing Zweig, who also lends his name to the protagonist's infant son, also unnamed in the original source material).

Fernand, a relative of Lisa's mother and eventual husband, is turned into the completely unrelated "Mr. Kastner", with the family moving to Linz rather than Innsbruck.

She meets him only one more time, many years later, at the opera, at which she promptly loses her present lover in favor of spending a fourth night with the writer.

Further divergences include a more prolonged "first encounter" between the two lovers (taking them through stagecoaches, fairs and ball rooms rather than simply cutting to the long-waited sexual encounter), revealing the disease that kills Stefan Jr. and Lisa to be typhus and ignoring Lisa's tradition of sending Brand white roses every birthday.

In fact, Brand's literary equivalent can only faintly recall Lisa after reading the letter, and there's no significant event past this.

(It was one of a series of "prestige" films from that studio which were financially unsuccessful, others including Secret Beyond the Door, Lost Moment, Another Part of the Forest, and Ivy.

Tim Dirks of Filmsite has listed it among the 100 greatest American movies of all time, and the film holds 100% approval among 22 critics on Rotten Tomatoes.