This act is central is unveiling more details about Mary's lost state of mind and portraying the first real confrontation between her and the men, which the previous acts have been leading up to. It is also important in confirming Edmund's diagnosis, that he has the potentially fatal disease of tuberculosis.
The act opens with Mary and the servant Cathleen talking together, although it is clear that Mary derives more comfort from her own thoughts than from Cathleen's conversation. The symbol of the foghorn, prominent throughout the play, is in evidence again here. It has connotations of being lost, of wandering alone in one's own personal fog of dreams and memories, which is very applicable to Mary (Edmund also remarks on it in the next act). Later Tyrone and Edmund return from town, where Jamie still is, apparently carousing and consorting with prostitutes as is his wont. The men realise that Mary has been taking dope and openly confront her about it but Mary simply retreats further and further from them; a process that will continue to the end of the play.