after the lecture we the audience soon find out that the man is the doctor in the story as he meets up with the sister in law afterwards. he drives them to his deceased lover's old home where she had died. he plans to kill the sister in law in the same way his lover died (falling out of the second story window). he tells the sister in law that he's considering buying the house and wants to take a look at it in order to get rid of any suspicion she might have. Once at the house and entered he insists on going upstairs which they do so. They enter the deceased's lover's former bedroom and the doctor then reveals to the sister in law what his real intentions are.
(I suck at describing things and i hope i'm doing this right)