"Yeah." The Doctor's jaw tightened. "Right. Going back in, Jack."
Jack just nodded, and shut his eyes again. If he thought about it too much, the fear and horror was going to overwhelm him and that wasn't going to help anyone.
The Doctor gripped Jack's shoulder comfortingly for a moment, then put his fingers back on Jack's temples. He went in more carefully this time, anxious not to damage anything, or disturb any evidence of what the Master had done.
Jack sat very still and just tried to keep his mind relaxed and quiet, and not think about anything in particular.
At last, the Doctor pulled back again, gently, looking frustrated. "I can't work out what's going on in there. It's obvious he did something, but I can't find anything out of place, or added, or deleted or. Anything."
"Fuck," Jack said, finally slumping and holding his head in his hands. It still hurt like hell.