The Doctor was worried. He could sense a great sickness within the TARDIS and it bothered him. He tried to track the sickness to its source but every time he thought he was close it would suddenly go away like it had never existed. It took the Doctor awhile to figure out Martha was usually nearby when this happened.
Whatever was causing the TARDIS to be sick had to fixed. The Doctor didn't think he could stand it if the TARDIS died again. It had been bad enough the first time! The TARDIS was all he had left of home. Plus with Rose's loss the TARDIS was even more important than ever before.
This morning the Doctor felt the sickness rising again. It was something he could sense and almost touch. This time he was more careful about trying to find it. He pretended he didn't sense it. He wandered around the TARDIS slowly like he had no real purpose. he went in and out of rooms. Sometimes he stopped to look at things. Slowly he made his way almost randomly towards the sick feeling.
It took a long time to move forward and still act casual but it seemed to be working. The Doctor found himself close to the laboratory. He paused near the door. it was open and he could inside. Martha was in there! She had her back turned and didn't see him. The sick feeling was very, very strong now and it came from Martha's direction.
The Doctor froze. Gently he probed at the air. Something was there and it wasn't his mind or Martha's. It was an evil presence. It meant to do great harm.
The Doctor probed harder. Too hard. Suddenly the evil thing was gone and it was just the Doctor standing there and Martha innocently working in the laboratory.