Two major things happening in this episode.
The first is Amelia Pond meeting the Doctor. Intriguingly, he arrives at the instant she is praying for help. Being a child, she doesn't fully understand who the Doctor is, but she knows that he is good - and hopes to encounter him again.
Years later (now referring to herself as Amy) she does meet the Doctor again. Now an adult, she is capable of more fully understanding the Doctor, the Tardis and all that involves ... well, at least as capable as any human being can be.
The other big thing is Prisoner Zero. Even the name suggests the epitomy of evil. Not prisoner 35784, or even prisoner 5. This thing has been evil for as long as there has been evil.And though usually unseen, it can disguise itself in the form of earthly things.
And here's where it gets even more Jesusy. Through the brilliance of the Doctor, and the efforts of mere mortals in "spreading the word", the world is saved from the incineration Prisoner Zero would have caused.
Back to Amy. In awe of the Doctor, she abandons her home, her job, and her earthly life in order to follow the Doctor on a far more interesting adventure.
Like many of the Doctor's companions, Amy leaves everything immediately. No concern for telling anyone where she has gone. Even the small matter of a wedding in the morning (her own wedding!) doesn't stop her from instantly taking off on an adventure. Following the Doctor through time and space.