There are many heart-wrenching, tear-jerking moments in Series 2's The Girl In The Fireplace. This moment is not one of them. Rather, it's a moment of pure heroism. The Doctor knows that if he travels through a time window to save Madame de Pompadour from the attacking clockwork robots at the Palace of Versailles, he will be trapped there forevermore, unable to return to the future space vessel and Rose. Oh, and Mickey. But he crashes through that mirror-like window anyway. Because he's the Doctor, and the Doctor always does the brave and kind thing, regardless of the personal cost. Of course, the Doctor does find a way to get back in the end, but that turn of events in no way invalidates the selflessness of his sacrifice - he had no way of anticipating that there would be a way to return when he jumped on that horse.