Fanny By Gaslight was one of Gainsborough Pictures romances starring its greatest stars, Phyllis Calvert, Stewart Granger, and James Mason all in their salad days. It's a Victorian soap opera with a lot of Dickens like class consciousness thrown into the mix.
The best works of Charles Dickens like David Copperfield, Nicholas Nickleby, and Great Expectations have the common thread of a young man of limited means making his way in the world who with a combination of hard work and good circumstances comes out on top at the end of the story. Fanny By Gaslight is just that kind of a story, except that Dickens would never have his protagonist be a woman. But Fanny Hooper as played by Phyllis Calvert is as good a Dickens hero as you will ever find.
When Calvert returns from boarding school her father, John Laurie, is killed in a fight ejecting a drunken James Mason from his establishment which is just this side of a brothel. When he dies she finds out that Laurie was not her real father, that she is the daughter of a prominent politician Stuart Lindsell. She's taken into his house as a maid. Calvert also makes the acquaintance of rising young politician Stewart Granger who is a protégé of Lindsell and Granger falls big time for Calvert.
Eventually all this becomes known about Calvert's background and it leads to an inevitable climax between Mason and Granger. How it gets to that point is the crux of the film.
Several incidents from the 19th century are used. The sex scandals are pieced from those involving Charles Dilke and Charles Parnell. Lindsell's suicide, jumping in front of a train is a recreation of the death of William Huskisson killed accidentally though by George Stephenson's newly invented locomotive.
Calvert and Granger are a winning pair of lovers and James Mason is one hateful aristocratic villain, a privileged man who lives to enjoy his privileges at the expense of others as Phillip Barry said.
I was surprised at how well Fanny By Gaslight holds up today. In fact the Hays Office had it banned from the USA for a while. Maybe that's its secret.