Theory:Doctor Who television discontinuity and plot holes/The Underwater Menace


 * Why are some of the Fish People more "fishy" than others? On some you can see they have scales and fins, but on others, they look like humans with goggles on!
 * Stages of mutation/chang'e? Also, as their adaptation is a surgical procedure, there is no reason they should all be fitted with exactly the same set of implants, particularly since some of them (the less adapted ones) are seen in the marketplace. Quite possibly they are given different configurations of implants to carry out differently allotted tasks, some requiring greater amphibiousness than others.
 * All the above, not to mention that their conversion is based on 1968 technology (albeit Zaroff's), and while invasive is thus probably pretty low-tech cybernetics by "normal" Doctor Who standards. Their shoddy and inconsistent appearance is, ironically, probably a fair reflection of what they might look like, given the conditions (Damon himself declaring that their "gills" are plastic prosthetics). Most of their "fishy" touches are probably cosmetic rather than genetic alterations, perhaps intended to depersonalise and humiliate the slaves into forgetting who they once were and accepting their lot.


 * When Zaroff fires his pistol (an ordinary, non-futuristic 9mm) at the end of episode three, he quite clearly does not: there is a sound effect, but no smoke, recoil, or discharged cartridge.
 * Production errors aren't plot discontinuities, any more than the fact that his 9mm is made up of scanlines on a cathode ray tube rather than 3-dimensional metal is.