Hester and Dimmesdale agree to flee with Pearl to Europe. As Hester makes plans for them to leave on a ship bound for England in four days, Dimmesdale feels changes coming over him, including the urge to speak blasphemously to strangers. He encounters Mistress Hibbins. She suggests they go to see the Black Man.
Dimmesdale's decision to flee has changed him, and even Mistress Hibbins recognizes his newfound freedom. But note that while his repression is breaking down, he doesn't act on it.
At home, Dimmesdale tells Chillingworth that the "free air" outside has done him so much good that he no longer needs the help of his medications. Chillingworth suspects instead that Dimmesdale talked with Hester, but feigns relief that his remedies have finally helped restore Dimmesdale's health.
Dimmesdale takes charge of his own identity and well being by dismissing Chillingworth, but to do so he lies. Chillingworth lies right back. Not everything has changed.
Dimmesdale throws the draft of his most important sermon into the fire and starts from scratch.
Dimmesdale's wants to start his sermon, and his life, anew.