Doctor Who has unsurprisingly occasionally had Quatermassian overtones (where’s that mash-up?!), but this is one of the most obvious. Great stuff, again. Seriously one of the best runs the Doctor ever had. Miss the Brigadier, though.
One of the few six-parters that doesn’t lag much, either, maybe because the first two parts are a discrete prologue to the latter four.
F/X-wise, man, they’ve sure come a long way from the guy with bubble wrap on his arm in Ark of Space.