As mid-story openings go, I think Part Three of "The Mermen's White Death" contains a couple of nice surprises. The first being that the Fish-men are actually part of some long-forgotten chapter of the Axis Powers, and resultantly are carrying weaponry which even sixty years later is still considered advanced.
Whilst the second is the appearance of one of my favourite super-villains, Carcharodon; a walking man-shark who also originates from the Nazi's vile experiments during the Second World War. Indeed, in the future, this former guardian of the Fuhrer predominantly provides me with an excuse to feature some lengthy 'flash-back' sequences to the war days, on account of the creature's constant reminiscing.
Its Bearman verses Carcharodon, with Controller Gardner's life at stake...