Encountering a Dalek with behavioural issues, the Doctor Et al. Are miniaturised and put into its tissues (inside it at least), in order to find out how this mutation evolved, and then subsequently and ever so naively resolved. It's a bit not like but very similar to Fantastic Voyage or Innerspace, though it's engaging enough to keep you in your place, and we can see that this Doctor's mind set is cantankerous, with extremely strong shades of being stubborn and quite rancorous. Hardly original though!