What is the main conflict in The Comfort of Strangers by Ian McEwan?

The Comfort of Strangers is the story of Mary and Colin, an English couple in Venice who, during a monotonous visit to Venice, meet Robert and Caroline, who offer their hospitality. Robert's brutal machismo, as well as Caroline's submission and her severe injuries always color this hospitality with uneasiness. Caroline and Robert's final bizarre act is to drug Mary and give Colin the choice to allow Robert to do anything to him and save Mary's life or let her die. Colin chooses to save Mary, and, when she awakes, he is dead.