Although we do not know the details of why or how, we do learn from the story that for some reason, Young Goodman Brown has agreed to make a trip into the forest on this particular night because on this night, there is to be a meeting in the forest. That meeting is actually an evil ceremony at which converts give their souls to the devil. Goodman Brown is a newlywed. He and his wife, Faith, have been married only 3 months and prior to his making this statement, she asks him to remain at home "on this one night." She suspects that there is something amiss with the mysterious meeting he has agreed to attend. We also learn that the meeting is in the forest and that it is to take place on this night only. This foreshadows the evil that will be revelaed as the story unfolds.
In responding thusly to his wife, Young Goodman Brown is trying to comfort her, assuring her that it will be ONLY on this night that he must "tarry away from" her but from that point on, he will not have to leave her and will stay with her. He sees her innocent face watching him, with her pink bows in her hair, as he departs.
Later in the story, we find out that the meeting in the forest is a gathering of devils and evil people, and many of the townspeople are in attendance, including the minister and the pious Goody Cloyse -- even Old Goodman Brown, Young Goodman Brown's father, is there. The worst part of the ceremony, however, is that Young Goodman Brown sees his wife Faith, one of the converts. He hurries home, and from that point on, he trusts nobody, not even Faith, believing them all to be like him, evil at heart. It is uncertain as to whether he really encountered the devil and his crew in the forest or whether it was only a dream. Dream or reality, however, it becomes his reality and for the rest of his life, he trusts no one, believing everyone to be an evil, dark sinner.