John Smith, the slightly odd new teacher at the Farringham School for Boys, is not who he appears to be. He is actually the Doctor, but not just in any old disguise - in order to hide from some aliens chasing him and Martha Jones across space and time, he uses the chameleon arch to change himself into a human, single heart and all, with nothing left of the Time Lord but a fancy pocket watch and strange dreams about adventures in a magic blue box.
Despite these precautions, the aliens do find their way to England in 1913. When they threaten the school, Martha makes the crucial decision to tell Smith of his true identity and encourage him to revert back to his true self in order to save the day. There is only one complication: John Smith has fallen in love with the school’s nurse, Joan Redfern. He doesn’t remember anything about this Doctor character, but he’s pretty sure that his other self won’t have the same feelings.
As it turns out, the feeling is mutual. Joan doesn’t want this other man that John Smith has become, even though the Doctor assures her that John Smith’s essence is a part of him. She rejects his offer to become a companion because he is the one who brought death and destruction to Farringham - the school wouldn’t have needed saving at all if the Doctor had not tried to hide from the aliens in the first place. Instead of facing the threat and dealing with it, he hid from it, and Joan calls him on it: “He was braver than you, in the end, that ordinary man. You chose to change, but he chose to die.”
Jesus was braver than all of us. We choose to change our minds and hearts back and forth all the time to and from the right path. We choose to hide from the things we should do, and to do the things that bring death and destruction even when we have the best of intentions. He chose to die to make up for it.