What is the main conflict in The Changeling by Thomas Middleton?

Alibius is a jealous old doctor who is in charge of a private lunatic asylum. He is married to Isabella, a woman much younger than himself, and he is worried that when he is away another man may usurp his position. He therefore instructs his servant Lollio to prevent any of the visitors to the asylum, who may include young nobleman who come to gawk at the inmates, from seeing Isabella.