The Adipose are gone, the world is safe, and even though the Doctor is never quite ready to go back to the TARDIS anymore, it wouldn't be fair to Donna to give her false hope.
There's no way he can safely bring her along.
But it's not the first time she's helped him, and maybe it wouldn't hurt to let her take a little peek ...
"If you'll wait out here for just a minute," he says, and slips inside. But of course she follows him through the door -
"Oi! Why've you got the Prime Minister chained up in your TARDIS?"
