Check out the summaries linked below. A few of the highlights:
Dimmesdale becomes even more powerful in his sermon. He wants to confess his sin, but only tells the church what a horrible liar he is and how he punishes himself. The parishioners thank him for begin such a model of holiness.
That night, Dimmesdale goes to the scaffolding where Hester was seen at the beginning of the book. He stand there, unable to move, wondering what would happen if he stood there till morning when everyone could see him. Hester and Pearl soon join him on the scaffolding--they were at the dying Governor's house. When the three hold hands, Dimmesdale feels an electric warmth through him. Pearl asks Dimmesdale to stand with them like this at noon, but he refuses her. At that point, a meteor flies through the sky, forming the letter A. The light reveals Dimmesdale with his hand over his chest, Hester with her letter A, and Pearl as a link between them.
Chillingsworth is there, sees the three of them, and takes Dimmesdale home, blaming his odd behavior on his "sickness".