It's the mid-nineteenth century in Glasgow. The upper class Smith family has just purchased the house at 7 Blythswood Square, of which the eldest of three daughters Madeleine Smith approves, as while she is being courted by William Minnoch, the man of choice by her stern and critical father, family patriarch James Smith, her basement bedroom provides easy access to the servants' basement entrance and allows her clandestine rendezvous with poor Frenchman Émile L'Angelier, the man she truly loves. In the progression of these relationships, everyone becomes frustrated: Madeleine's parents and patient William for no official announcement of an engagement, and Émile, as, although he and Madeleine are instead the ones engaged albeit secretly, she is hesitant to tell her parents about him, which relegates him to sneaking around that basement entrance like a common thief. Things for Madeleine change when she comes to the realization that Émile wants to marry her solely to raise his social and by association financial standing. As such, she accepts William's proposal of marriage while tries to break off all ties with Émile, which includes asking him to return her love letters as if they came to light would place her in a difficult position especially with her father. But when Émile dies all of a sudden, it determined by arsenic poisoning, Madeleine, upon discovery of her and Émile's relationship, becomes suspect number one as being his murderer.