The cold, cold night Miss Maudie's house burned, Boo Radley left his house, slipped into the crowd, and put a blanket around Scout's shoulders to keep her warm. After the excitement of the fire dies down and the children go home, Scout realizes the blanket is covering her. When it becomes apparent to Jem that Boo had done this, he is so shocked he tells Atticus all that has happened in regard to Boo, which stuns his sister:
Jem seemed to have lost his mind. He began pouring out our secrets right and left in total disregard for my safety if not for his own, omitting nothing, knot-hole, pants and all.
Jem then says Boo is probably crazy, but he has never hurt him and Scout, even when he could have. Atticus replies, "Do not let this inspire you to further glory, Jeremy." Once again, Atticus is telling Jem not to disturb their neighbors' peace, to leave Boo alone. Jem agrees and Scout "watched the spark of fresh adventure leave his eyes."
Jem has seen a new side to Boo Radley, but he understands him less than ever as he tries to reconcile Boo's kind actions with his terrible legend in Maycomb. It also spooks Jem that Boo had come out of his house and had been so close to them without their knowing it.