He will knock four times | Doctor/Master (by KatrinDepp)
The Master and the Doctor’s relationship is the most intriguing and complicated of anyone’s relationship in Doctor Who in my opinion. The Doctor wants to stop him, but also just wants him to live and use his greatness for achieving good. The Doctor wants to go on knowing there is another Time Lord in existence and that he doesn’t carry the burden of time all on his shoulders. Then when you meet the other Time Lords in “The End of Time Part 2” you understand much more as to why the Master is the way he is and actually sympathize with him.