The film deliciously stays true to time (the mid-19th century) and place (mostly Paris) and social mores (women sacrificed, men seduced) -- and without a single vampire, only a domineering aunt who makes Jane Eyre's aunt look like Aunt Bea in Mayberry. Therese is dumped on her aunt by her father, and she is put to bed with her cousin Camille, who suffers from a lung condition and from his mother's coddling.
Therese's father dies, leaving her a small sum of sous. Mdme. Raquin insists that the cousins marry with Therese as Camille's guardian angel. They move to Paris so that she can open a little shoppe with Therese as her dog's body. Camille is well enough to get a job clerking. At the office, he meets Laurent, the son of a former neighbor of the Raquins' from the country. When Laurent comes to the Thursday night socials to play dominoes with the Raquins and their circle of friends, he takes one look at Therese, at those big eyes and those longing lips, and he succumbs. Ditto, Therese.
It's no surprise that those two will go at it like rabbits. One of the opening scenes shows Therese rubbing against the grass by the river as she stares at a bare-chested scyther; another scene shows her lying like a tombstone beneath her clumsy husband. When the trysting twosome decides it's time to give Camille the heave-ho, they have to live with their actions -- or die. It's a 19th-century question of morals, mores and morale.
As director, Stratton does some dandy editing of that story, trusting the audience to fill in the symbology, for example, when Laurent tells Therese that he painted her shadow into his portrait of Camille. Cinematographer Florian Hoffmeister presents Paris as cold and grey and wet, a dead cod of a city, and films faces close up. The lighting is admirable throughout.
So is the acting. The supporting cast includes the familiar faces and voices of Shirley Henderson (Moaning Myrtle in the Harry Potter films), Matt Lucas, and Mackenzie Crook. Elizabeth Olsen is a perfect Therese, all eyes and corset, and Oscar Isaac is beautiful as Laurent, better enveloped than as Llewyn Davis. Tom Felton (Draco Malfoy in Harry Potter) is believable as Camille. Jessica Lange steals the show as Mdme. Raquin, especially when speechless and venal.
"In Secret" manages to be both historically interesting and a bit asinine all at once, and very satisfying.