Coat-check girl Bessie MacGregor (Hope Hampton), is struck by the car of wealthy society woman Mrs. Templeton Orrin (Theresa Maxwell Conover), who takes Bessie into her home while she recovers. Mrs. Orrin's brother, J. Warburton Ashe (E.K. Lincoln), says he loves Bessie, but when she learns he does not mean it, she flees the home, heartbroken.
Unable to find work, Bessie collapses one day in the boarding house in which she is staying. The landlady, Mrs. Flaherty (Dorothy Walters) and another boarder, Tony Pantelli (Lon Chaney) start to nurse Bessie back to health.
Ashe, realizing he was wrong in his treatment of Bessie, goes on a trip to England to forget about her. During a hunting expedition he finds a mysterious chalice that some believe to be the Holy Grail. Mrs. Orrin urges her brother to return home to help locate Bessie.
Seeing Bessie needs medical care, Tony tries to raise money by stealing the chalice. The police later recover the chalice in a raid on a pawnbroker's shop. News of the cup's mysterious healing powers, and the way it glows in the dark, reaches the newspapers.
After Bessie tells Tony the story of the Holy Grail, he again steals the chalice, this time to cure Bessie who makes a recovery, but Tony is caught and put on trial for the theft. During the trial, Bessie and Ashe are reunited and when Ashe refuses to press the charges against Tony, he is acquitted. Later, the pawnbroker, now in Sing Sing prison, confesses that the mysterious glow was from some radium he had placed in the chalice.
Light in the Dark was filmed in New York.
A review of The Light in the Dark in Moving Picture World noted: "In introducing the new process of color photography, Associated First National has made doubly secure an offering that from the standpoint of material and treatment promises to give wide satisfaction ... Lon Chaney has the type of role in which he has proven exceptionally skillful. His is a real sympathetic contribution."
The Variety review said, "If its story possessed half the merit of its technical equipment, it might have proved a world-beater. It doesn't, so it isn't ... Mr. Chaney is a somewhat more kindly crook than is his wont, and Mr. Lincoln struggles along in the fat, but unconvincing hero role."
The original seven-reel film was re-edited into a condensed 33-minute version known as The Light of Faith, that was circulated to schools and churches in the 1920s. A copy of the film is in the George Eastman House Motion Picture Collection.