Previously in the story, Nat was banished from Wethersfield under the threat of punishment should he return. Kit is on trial and realizing that the situation is hopeless because, besides her uncle, no one will speak up for her. Just before the end of the inquiry, Nat arrives with Prudence in tow. He has risked his own safety and freedom to help Kit.
The magistrate quizzes Prudence about her ability to read and write. All the while Goodwife Cruff, Prudence’s mother, is railing against Kit as a witch. Prudence is able to show the magistrate that she is able to read and write and explains that Kit taught her. As a result, all charges against Kit are dropped. Nat leaves as soon as it becomes apparent that he has been recognized as a banished person, however the magistrate is willing to let him go since he violated his banishment in order to achieve justice for Kit.
By showing that Prudence can read, he shows that Kit was teaching her to read (and not doing witchcraft). This saves her from a harsh, possibly deadly, punishment.
His mysterious person that that saved the day was Judith. the way she saved the day was that she said she wrote in the book and kit only wrote her name once and she copied it. the second think that she did was that she said that kit taught her to read. they asked her what she read she said the bible. so they thought that the devil wont work agianst the devil so thats how kit got saved. i hope this answered all your questions.Good luck!