Martha is having a rough time, The Doctor knows this and plans to surprise her on Christmas. (Set during Blink, prequel to From Bad To Worse as it explains the story of something mentioned within the story, it can be read as a standalone though.)
Everyone knows that The Doctor is brilliant at getting into trouble. So when he is having a bad day you better start praying for the safety of the universe, Right?
The Doctor goes to Martha after the events of Having a Bad Day, so she can help him recover. Things start to get complicated and the fact that it's going downhill with Donna while he's still recovering isn't exactly helping either. Sequel to Having a Bad Day