This is the 6th novel published by the BBC to feature the 8th Doctor.  His companion is Samantha "Sam" Jones.

Every now and then, Sam found herself thinking of the Doctor as a set of responses, not a man - half-man - person - at all.  Maybe, just-maybe, that was the only way a poxy human mind like hers could come to terms with him.  As an equation, rather than a living being.  A function of the universe, whose purpose was to (a) break into places and (b) break out of them again.  It didn't matter whether he was dealing with a cast-iron padlock or a bunch of genetically engineered toucans.  Security devices would take one look at him and give up.

The wall of the "Lost City" hadn't been far from the outhouse where the Doctor had found the biodata machine.  He'd strolled right through the City archway without a second thought, head in the air, hands behind his back.  He'd made straight for the central pyramid, sniffing disdainfully at the smaller buildings around it.  'Shoddy workmanship,' he'd mumbled, more than once.

They were inside the pyramid now.  Sam had seen a lot of corridors over the last few months, but the passages here were something new.  No vent shafts or strip lighting, for a start.  Lots of quivering shadows, lots of flickering torches.  More like the cloister room of the TARDIS than, say, the connecting tunnels on board the Quetzel.  These corridors were more...

'Corridory,' Sam suggested, accidentally saying it out loud.

A couple of metres ahead of her, the Doctor stopped at a three-way junction.  He seemed to have heard her, for once.

'Yes, they are, aren't they?' he muttered.  He surveyed the junction for a moment or two, then licked his finger and held it in the air.  'Purity of architecture.  Most corridors are built to be functional, but this one's supposed to give the impression of being a corridor, judging by the feel of it.  Are you psychic, at all?'

Sam suddenly realised the Doctor had turned to face her.  She saw big blue-green eyes in the half-light.  Staring, not bothering to blink.  You could tell, by the look on his face, that the Doctor thought his eyes were full of madness and poetry.

They weren't, though.  In a previous life, this had probably been his best hypnotising stare, but his face was built differently now.  Sam knew all about the Doctor's previous lives, the other bodies he'd lived in and lost over the years.  She also knew that "this" Doctor, "her" Doctor, still didn't really understand what he looked like, or appreciate the impression he left on the rest of the universe.  How long had he been walking around like this?  And he hadn't figured out who he was as yet.

Just for a second, she felt sorry for him.  Because he wanted to be a force of nature again, he wanted to be the incredible escaping equation all the time, but instead he was trapped in a half-human body with a baby face and floppy curls.

'Psychic?' Sam queried.

The Doctor broke off the stare.  'There's something here.  Something trying to make contact.  I can almost feel it...'  He punctuated the sentence by jumping up and down.  Testing the gravity, maybe.  'Beneath our feet.  Something beneath our feet.  Throwing out tendrils.'

'OK, let me try and translate this into English.  Somebody in this pyramid's trying to make psychic contact with you, is is that what you're saying?'

'It's not a pyramid.  It's a ziggurat.'  A new expression materialised on his face.  It took Sam a few moments to identify a look of pure hurt.  Like a child whose parents had just told him that he smelled.  'lt doesn't want to talk to me.  Every time it comes close, it pulls away.  It's trying to make contact, but it's...'

The sentence ended in mid-pontification.  The Doctor moved, faster than Sam could follow.  With one smooth motion, he turned, and leapt back down the passageway towards her.  A second later, he was standing with his back pressed against the corridor wall, pulling Sam towards him.  His hand was clamped across her mouth before she'd even managed to open it.

There was a second or two of absolute silence.  Then there were footsteps.  Human footsteps, by the sound of them.  Around the corner.  Getting closer.