He had known even before Jack entered that something was wrong. He was radiating pain and fear like a beacon from down the corridor. So he had laid down his sonic screwdriver and was already getting up, looking towards the door, when Jack entered.
He watched him shaking, standing just inside the door of the TARDIS, his arms wrapped tightly around himself. He took off his glasses and approached, as Martha entered behind him, saying something about Jack not getting any rest. They had been with Lucy Saxon, getting her ready for transport. She hadn't said a word since shooting her husband, but suddenly she said something to Jack, and now he was like this.
He watched Jack back away from them and bump into a pillar, sliding down it until he hit the floor, all the time shaking. He seated himself on the floor as well. How had he explained it to Martha? "It's not really a surprise. The human mind isn't really built to deal with what he's had to face … well, no mind is, really." He watched Jack staring at him in wide-eyed anxiety, trying to speak without success. He took his hand. "It's all right. Of course I'm going to help."
Martha wanted to know if Jack was having a breakdown, to which the answer was of course, yes, or very close to it. He didn't have the energy to help him, not really. He was already exhausted, depleted. But it couldn't wait. He had to try. He told Martha to make sure they weren't disturbed no matter what, and then he reached out to help.
The journey through Jack's mind had been a nightmare of mutilation and death, and the effort had exhausted him. The Master's presence overshadowed everything, words like 'freak', 'filth', 'dirt', 'monster' echoing through the mindscape. Jack's terror of the Master, firmly suppressed in the physical world, was here an overwhelming reality. Underneath all of this the Doctor had found a century of loneliness and confusion, an image of the TARDIS disappearing from the Game Station, and his own stupid words on Malcasiro so firmly intertwined with the Master's they were barely distinguishable: "you're wrong … hurts to look at you … it's instinct." How Jack had trusted him enough to let him in the Doctor could not fathom. It was undeserved. It was humbling.
He opened his eyes and stared up at the ceiling of his room, then lifted a shaking hand to his face and wiped away sweat. He looked at his damp hand, frowning. That's not good. Should go back to the medical unit and check it out. He didn't move. Maybe later. He closed his eyes.