This movie succeeds where many typical Japanese monster movies fail. The plot is thick and evolves into some sort of spy story about a renegade, long lost naval officer, whose daughter is in the custody of his former commanding officer who -in turn- has kept silent about him for 20 years. On top of that, some mysterious submarine empire wants to conquer earth, and demands that said lost officer be handed over, in addition to the latter's brainchild, a powerful submarine. The renegade is now assembling his submarine on a remote island with a garrison of left over soldiers & natives, hoping to win WWII in the 60's. Such submarine ( the 60'ish version of GOTENGO in final wars, again coming in the 70's Tsuburaya production "I-zenborg" ) is the last hope for humankind. The plot -as said- is unexpectedly solid, a cut above most monster movies, and adds drama and spy story in a credible fashion. Of course that's from the Godzilla crew, so we have to adjust our expectations period & genre-wise. Trademarks abound, EG the submarine invaders look like the "savages" on the Mothra island: they just wave spears & dance around pseudo-Egyptian monuments.Overall a cut above the usual man-in-suit smashing cartonbox miniatures movie and without too many bond movie cloned moments.
