George Sanders plays an unusual role here - basically henpecked bachelor living with 2 eccentric sisters. He develops an unexpected romance which is increasingly resisted by one of the sisters. He finds himself unable to separate himself from this sister and loses his love - and interest in life basically. He ends up murdering the troublesome sister and too late - confesses to the crime. All somewhat pedestrian thus far.
Then, the ending! I'm shocked at the the reviews that don't like it - it's what makes the references to Hitchcock reasonable.