Siskoid and Ashford discuss The Aztecs by John Lucarotti, a true historical where the Doctor admonishes Barbara Wright about "messing" with time. This first, on air, T.A.R.D.I.S. team are separated quite a bit, covering more ground of this landscape where the Aztecs resided.
Was Barbara right for trying to change this culture with her relatively modern, English ways? Perhaps stepping back and allowing History to take its course is not the right thing to do?