The Doctor ambled along behind, trying to figure out where in his personal time-line the Master could be from. He recognised him as the Master, which meant he'd got a new set of regenerations from somewhere...
"Well," he said aloud, "I'll be sure to keep an eye on him, then." He leaned against the wall, smiling despite himself. This new Master's childish antics were almost endearing. "How far ahead are you? Have I changed much, where you're from?"