Imagine

For rude_not_ginger for the song drabble meme. [This might be technically considered to contain spoilers for Waters of Mars]

Dennis refuses the Doctor's invitation to travel only once. He's eighteen at the time, being trained up as an Unspeakable in London's Department of Mysteries. They won't let him touch the good stuff yet -- there may have been an explosion or three, because Dennis has never yet met a button he wouldn't press just to see what it did -- and so, when the TARDIS groans its way down into the belly of the Ministry, it appears surrounded by chalk-boards covered in runes and arithmantical equations.

"Well, that's wrong for a start," the Doctor says, grinning hugely in the doorway.

Dennis just smiles and holds out the chalk and the Doctor bounds to the boards, slipping his glasses on and cheerfully slashing lines through hours of work, scribbling notes into spaces in the margins.

"You should get bigger margins," he tells Dennis, and then grins even wider. ( I just said that to Fermat. )