I've recently introduced my girlfriend to Doctor Who, we've been taking non chronological trip through 9, 10, and 11. Anyway, she asked me if there were any episodes where the only human is the Doctor's companion (or companions as the case may be). Any other characters would have to be aliens. As far as I can tell, this has not happened in the new series. Since I've not seen the classic episodes I couldn't comment about them, so I'm deferring to the experts.