"No idea." Since Martha didn't push him for more information, he didn't volunteer any. He'd rather not talk about the things he'd done after he'd lost Donna, or even how he'd lost Donna in the first place. For the time being, he'd let Martha think that he'd simply dropped Donna off and gone on about his everyday business as usual. Plenty of time to get into the messy details later.
"Trouble is, this place doesn't seem to be like any parallel dimension I've ever seen. There's a man here called Galen--terribly rude, extremely pompous, bit like dealing with the Time Lord on the High Council--who believes this is a pocket dimension, and I'm inclined to agree with him, at least on that count."