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 1970s 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 merely plastic prosthetics). Most if not all 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.


 * Alternatively, they may even be cosmetic touches added by the older, more established Fish People themselves. Damon says that some people "get quite upset when they learn they're to have the operation." Atlantean society, whilst far from perfect, does not appear to be an absolute totalitarian dystopia- note that whilst they may embrace human sacrifice, and compulsory surgical alteration of prisoners, nevertheless, Thous, established leader of the city state, is appalled at Zaroff's suggestion of slaughtering the rebellious Fish People. Some of the Fish People may well have been either volunteers, or, at least, tacitly willing to concede that converting their farmers for aquatic environments was a necessity for survival, and thus, chosen to embrace that new identity and dress the part.


 * 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.